Auto parts are pretty common. You can find them almost anywhere; salvage yards, auto auctions, car dealerships, etc. If you are having a difficult time finding affordable auto parts, you could try any of the previously mentioned places, or you could try one of the more unusual places listed below.
Charitable organizations known for their work with teenage boys often use cars as a means to rehabilitate and train boys to become better men. While it is more common for people to donate cars and parts to these "ranches," you may still find a couple of used and affordable car parts there. Generally, a reasonable cash donation is an acceptable offer to these organizations in exchange for the spare parts they are willing to part company with and share with you.
Privately-Owned Auto Garages That Are Going out of Business
Privately-owned auto garages often buy extra parts to keep on hand. Sometimes these garages end up with quite the inventory, and then they have to liquidate it all when the boss retires or suddenly passes away. If you keep your eyes and ears open, you may just find some very affordable auto parts among the inventory of these automotive businesses.
Junk Pickers (a.k.a., Collectors)
You have probably seen the popular cable TV show "Pickers." If you have not, it is about two guys who drive around looking for people and properties that are stacked to the ceiling with cast-off "treasures." If you really want affordable auto parts, you could become a picker for a while yourself. (You may even find an old car that is completely intact and get it for pennies on the dollar!) Although you probably will not turn junk picking/collecting into a full-time job like the guys on the show, you may have a lot of fun searching for the parts you need and bargaining with their current owner to get the parts at a rock-bottom price.
Believe it or not, pawn shops do deal in auto parts, especially if the parts are from rare or vintage cars. Since the parts have to be verified as functional parts before the pawn shop will buy them from the customers, you can be guaranteed that the parts you find there are in good working order. If, for whatever reason, the parts do not work, some pawn shops are willing to refund you part or all of your purchase price within a specified time frame.