I know about Fastenal* and Grainger+, etc. but if you're looking for something obscure, check out this place!
https://www.boltdepot.com/About/Default.aspx?nv=b
I needed an M6 1.0 coarse x 80mm hex socket cap screw for a little project in my truck (mounting a ram ball to the door pillar) and the first ones (bag of ten) I ordered from Amazon came as hex HEAD bolts. Fuckers. Sent em back and tried another seller (bag of 6). After ordering, I went looking around the net again to hedge my bet and found these guys. They seem to have literally anything you could ever need. They had the hex socket cap machine screw in every possible size, thread pitch, length and material you could imagine at ANY QTY. I was blown away.
* could not locate the cap screw at all, maybe I'm looking in the wrong place
+ min order was like 25 or 50 depending on material
P.S. I'll take pics and post a parts list when I get done for anyone else that may want to do this. There's an outfit that makes a steel Ram ball mount for Jeeps, but nothing for other Makes. I found a through hole Ram ball mount that was setup for a motorcycle (but it came with three different 8mm socket cap screws) and I needed a long 6mm bolt and some spacers to make it work in my truck.