I have a local retailer that sells nothing but bolts, nuts and washers (Bolt Supply House). Large warehouse with both customer-accessible aisles and counter sales.Last ones I bought were for seat mounts, which carry a mandatory strength criteria for collision safety. There is literally no fastener you cannot buy from them, $5 minimum charge. Cheaper than the dealer and super convenient.
You can order by the usual criteria or bring in the fastener and they mic the thread pitch, threaded and unthreaded length, determine the grade and material + finish, and two minutes later return with your fastener. Because of the minimum charge, you just add whatever to the order to total the $5 and you are done.