one of the best sources i have found, especially for kit bashing and spare parts, is E-bay!!!!!You can always find auctions for a " lot " of parts and complted/un-completed models. I once bought a NASCAR lot for 15 bucks. When i got it, it had 25 bodies, over 200 engine pieces, roll cages tires, etc...... great for scratch building and kit bashing!!!!!
I also buy alot of regular kits on ebay.
The only real trick is to know a deal when you see one!!!
Hope this helps.....
Goodluck and God Bless!!!!!!
Ron