How to find cheap parts
 Be realistic there, speed racer
One of the first tricks to saving money on parts is only buying what you need. One of the most commonly messed-up parts choice is brake components. For example, people buy race-type brake pads and the cross-drilled rotors in order to get that last 2% more braking power for their SUV that is driven back and forth to work in rush hour traffic. Race-type brake composition can actually dramatically reduce your braking force since they are designed to only operate in a very high temperature range. Its a big mismatch. Buy parts that suit your actual driving style. Many times, buying upgraded parts will only be a benefit if you are doing upgraded driving. Other common money pits are engine oil, gasoline additives, wheels and tires, alternator amperage, and air filters. Sometimes you need the upgrade, sometimes you don't.
 Know your quality brands, know your junk
In addition to knowing which type of part you need, it's wise to know what brands will offer the most bang for the buck. Many times there is no need to spend the extra money on the factory original part. In fact, many factory original parts are pretty pathetic pieces that were supplied to the auto maker by the lowest bidder. For instance, you might find that Gates belts and hoses are cheaper and better than the original ones that came on your vehicle. Other times you might find that the economy replacement part is really junk and should be left on the shelf. Knowing which ones work best is key to maximizing the quality of the parts you get for your money.
One of the best ways to find out which ones work best is to ask the parts dealer. This means a wholesaler, not a retail parts-store employee trying to get a bonus for upselling a premium part. Which parts have fewer returns for defects? Chances are they've handled every brand that comes through the door and they can attest to their quality first-hand. Asking on forums will get you widely varying results. On one forum, perhaps someone made a compelling argument for one brand because it worked for him/her, but the truth is somewhat obscured by the opinions of forum members and an inaccurate grasp of their experience. Does this poster have 10 years experience in a shop that specializes in your vehicle, or is it a shade-tree mechanic who just changed their own brakes last week and wants to chime in?
 The password is "undercar"
Most towns and cities have a wholesaler that goes by the name "something Undercar." When you go to a shop to have your brakes done, they call up their undercar wholesaler who then delivers parts in a few minutes to an hour. They of course tack on a huge upcharge and pass that on to you. Oftentimes these undercar wholesalers also supply your local chain parts stores. The bigger ones like Auto Zone or Pep Boys typically have their own supply hierarchy, but if the customer bats their eyelashes, often times Manny, Moe, and Jack will call up the undercar wholesaler and see if they can make a buck.
The beauty of it is that the wholesaler will sell directly to you without even so much as a blink of an eye. If you don't have a retailer's license, they probably won't deliver to you, and of course you have to pay tax, but who cares? Since most of these wholesale houses supply multiple retailers with parts, they also typically carry more brands in one location. They also keep a lot of stock. Did you ever go to a retail auto parts store and they tell you they can order the part and it will be in tomorrow? Chances are that the part is sitting on the wholesaler's shelf and can be delivered later. Skip the middle man and go straight to the wholesaler. You get your parts faster and cheaper.
These wholesalers become especially appreciated when it comes to expensive foreign makes. Mercedes and BMW are well-known for their high parts prices, but you can purchase $300 Mercedes parts for $75, and $60 BMW parts for $7, often in boxes that say "Mercedes" and "BMW", not off-brand replacements - factory original parts. There is no need to pay retail prices at chain parts stores, and certainly no reason to pay parts counter prices.
The added benefit for you is not only saving money, but the people who work at these wholesalers are in constant contact with hundreds of shops that do everything from exhaust, brakes, custom fabrication, restorations, paint and body. They spend 10 hours a day doing nothing but gaining experience with almost every facet of automotive repair and customization. As a rule they know their parts.
Other wholesale parts "chains" to look for are WorldPac, Wholesale Parts Direct, and 1-800-radiator.
 Head for the junkyard
Many parts can be purchased for pennies on the dollar at the local recycling yard. Not only does it save you money, but it saves another part from being manufactured new. Why buy a new wheel when there are 500 of them in the junkyard that just need a rattle can paint job? Some geographic areas are more conducive to salvage yard operations and others just are a little lean. Some places it's not much of a savings, while others are fun, inexpensive, and still operate on the good-ol-boy mentality. Wheels, axles, differentials, body parts, electronics, driveshafts, transmissions, gas tanks, hardware, and glass are things that are commonly available at junkyards. Many used engines and driveline pieces that they sell as running replacements are still in cars where they can be tested, driven, and heard running before you plop down the cash. Some even offer a 30-day warranty.
Another thing you might pleasantly find at junkyards is that the cost of buying a larger part can save you money. If you need a set of heads, chances are it's only a few dollars more to buy the whole engine. Then you have extra parts you can keep or sell to offset the cost. Save your pennies, save the environment.
You can also often find great freebies at auto repair shops. While it's not often acceptable to use worn-out parts, some customers want their tires replaced even though there is still good tread on the old ones, or new shocks because the ones they put on last month are too stiff. You'd be surprised what gets tossed. If you make a relationship with the shop manager or owner, chances are they'll be very happy that you are carting off their trash because it saves on disposal costs. Larger dealership service departments are great for this. Most of the replacement parts that the manufacturer covers under warranty get sent back to the source for testing or proof that it was failing, but some less-specific parts like axles, engines, gas tanks, transmissions, and electronic parts don't get sent back. Most of this bonepile gets picked through by the techs in the shop, but many times there are very large piles of neat stuff. Make a friend in the department, and have fun digging.
 Use online sources like Craigslist and Ebay: Buy early, time is your friend
If you know ahead of time what things typically break on your model of vehicle, or if you know you plan to upgrade in the future, its wise to have them ahead of time. Time is your friend when it comes to cheap parts. For instance, if you have a 7.3L Ford Powerstroke diesel, you know that you'll have to replace a glow plug relay and a cam position sensor at some point. Keep your eyes open for cheap deals on them and get them on your shelf so you're ready to fix it when it goes. If you plan on upgrading heads on your chevy 350 next month or next year, start shopping now. There is a good chance you'll find a real steal on parts instead of waiting until you're ready and having to pay full market value. Wait for the cheap deals, be ready by purchasing when it's cheap. When you need something, you usually pay full price for it. When you want something, you're more likely to pay less.
There are many online sellers who get a hold of a large lot of the same product and put them up for sale at really cheap prices. As long as you're dealing with a new-in-box product, you can be assured its the real deal. If it isn't the real deal, you are protected under Ebay's policy.
If you have some space in a garage or shed, stockpile some things. I have a search saved on Ebay that sends me an email everytime something gets listed that fits my criteria. For instance, I have one set up to email me any time parts come up that fit my cars that are under $10. You'd be surprised what comes up.
Anytime you have a few minutes, comb through your local paper, and especially Craigslist. Good deals go fast on Craigslist, so its wise to be ready to jump. You can even score free engines and cars.
 Using online coupons
Have you ever bought something online and one of the optional boxes during checkout is a space for a coupon code or promotional code? Do a google search for "coupon code." There are hundreds of sites that stockpile these passwords that you can input for discounts. Its all legal. They are just websites that allow you to search for published coupon codes.
 Talk to your local scrapper
I know that we all hate to see stuff get scrapped, but it's a fact of life. Get out and talk to the guys you see with "buying junk cars" signs on their trucks. They come across all kinds of vehicles, and those parts you need may be on one of them. He'll make a lot more for the parts than for the scrap, and you'll still get a great deal.