Where can I source replacement screws?

I was given several Mac laptops (MacBook, Powermac Pros, and iMacs) from a friend’s Windows repair/services shop; he does not work on Macs. They all need something fixed & replaced and I can do that. Someone opened them up to remove the ‘highly valuable’ 10 year old RAM. The laptops are missing most/all the back cover screws; the iMacs are missing most of the display T10 screws). Where can I purchase such screws? Too small/unique for Home Depot/Lowes. Perhaps a hobby shop (RC cars/planes). I will check with Micro Center & Frys. Any suggestions welcome.