Ace Hardware has a pretty decent fastener aisle. I don't know if they still use Midwest, but you can find just about anything there (except riv-nuts, which apparently don't exist outside of the internet). Home Depot and Lowes don't have as much "niche" hardware in their drawer layout. Our local Ace has antique firearm hardware, of all the thing... Belleville washers shouldn't be an issue. Their website is not entirely accurate on selection.
I worked for Ace Hardware's competitor 22 years ago and I'll tell you to go to Ace instead.
I hear you on McMaster Carr. I use them a lot for work. Not the cheapest, but the selection is amazing. I also use Essentra, Misumi, MSC, Grainger, and Fastenal a lot. Fastenal has the same issue as MMC. 17 years ago I was driving a drag car down a back road, completely fresh build, maiden voyage to test it out. I went to hit the brakes approaching a red light, pedal made two loud snaps and fell. I cut the wheel and went up into shopping plaza, rolled in circles until I came to a stop. Both studs holding the MC to the firewall had snapped (brand new, mind you). I had some hand tools, pulled one stud out, walked to Fastenal. $100-something dollars later I had a bag of 250 of the dang things. They wouldn't just sell me two. XD
