Frequently I've found folks buying new parts only because they are unsure where to find good used parts. The two below links are a couple that I use constantly.
Ever find yourself wondering if there are junkyards in your area?
Listing of many JY's
http://www.auto-salvage-yards.com/directory/index.html
Want to search the entire US for a specific part?
Listing of parts by yard, searchable on price, location etc etc.
www.car-part.com
Need a GM part number?
Once you find the drawing with the part you want, click on the drawing and the listing with part numbers appears
http://www.compnine.com/