I find that getting crappy corps figures on clearence at toys r us or wally mart is a good bet for screws, you just take it apart and you get a screw, extra o-ring and there accessories to boot.plus all the custom fodder parts.
Or try a hardware distributor called Brafasco, they have pretty much any kind of screw you would need. Puts Home Deopot screw Isle to shame.
they have tons of locations, look them up on the net. not badly priced I might add. I use them for work all the time.
