Without more info, I would just say the hardware store.
I've had good luck also with
http://www.loosescrews.com/index.cgi
http://stores.ebay.com/Retro-Bicycle...id=p4340.l2563 (he also has a web site, but I forget the name).
Also the WTB section he can be good if you have specific parts.
And old bike shops if you are lucky.