This forum is the biggest resource I know. Not necessarily for parts, but certainly the knowledge of where to find them.
I'll plug Sew-Classic.com for many of the more commonly needed replacement parts.
Singerco.com is good for free downloadable Singer manuals.
Don't overlook the manufacturers that are still around. Many are excellent resources for dating your machines. I'm surprised at how many still have their old records.
Rodney