I used to sell specialty nuts,bolts and moulding clips,etc,mace by a company called "W & E. ,they had a suitcase type display with samples of various fasteners and clips that we sold in bulk to body shops...not sure if they are still in bussiness--their prices were sometimes high,but they also had many OEM style fasteners no other brand had..
I dont know if McMaster-Carr stocks automotive bolts,but they have a wide selection of just about every type of hardware and fasteners ...
We just had a new Tractor Supply store open nearby about 6 months ago here--I like the fact they sell nuts and bolts by the pound,few if any hardware stores do any more--most of their other parts and stuff in the store is very high priced though,compared to other sources..I can buy a kevlar v-belt at a NAPA or other auto parts store for 5-10 bucks less than at Tractor Supply..
.their lines of clothes and boots are way out of my price range too-a regular show store can beat them by quite a bit...after pricing their oils and other lubes and parts,I wont be buying much there,the truck supply store sells bulk oils in 5 gallon pails for much less,the same brands too...you really need to shop around now,it used to be every stores rices were pretty much the same,not any more...The 2 times I have gone to Tractor Supply to get something,they were "out of stock"--so before I go 10 miles to find out they lack what I need next time I will CALL first!...
I have a place a few miles away called "NUTZ" that sells all kinds of nuts,bolts,screws,moulding clips,etc...only bummer is the guy has sparatic bussiness hours and is often not open ,when he's out on the road trying to increase sales to his customers who keep things in stock he sells--he goes around and takes inventory and keeps their bins filled ..seems like every time I go there he is not at the store!...but when you catch him there chances are good he'llhave what you are looking for,and gives you a good deal on them..