Do you want vintage parts (from old machines) or replacement parts (new)? I usually try to go to a local sewing machine dealership first (one that does repairs on sewing machines) so I can avoid shipping costs, which can add greatly to low-priced items. When that doesn't work, here are some online places I check:
http://www.sew4less.com
http://www.sewingpartsonline.com
http://www.vintagesingersewing.com
I also sometimes check eBay. Googling can turn up other places to check as well.
Lostn51 on this forum (Billy) may actually be the most efficient person to check with first.