My personal favorites are S-Lok fasteners made by Kerb Konus. They are a pain to deal with, I have to call them on the phone to order, but very cheap and the inserts are high quality. Here is the US distributor: http://www.precisionfastenersinc.com/

Easier to deal with, but more expensive sources listed below

PEM Engineering: http://www.pemnet.com/

McMaster: www.mcmaster.com