My go-to is my local Ace Hardware; I can generally get any size fastener I need, by the single piece.
You do need to keep track of bolt size / length / thread pitch / hardness, of course.
I try and stay away from what I call the 'mini' head size bolts; while the thread pitch is rated the same, the threads are thinner, and they'll cause problems when torqueing them (10mm water pump bolts come to mind). These are always in smaller bolt sizes - listed as M6 / M8 / M10, but the head / socket size will be smaller than the OEM. They'll also be flanged, and the flange will slope slightly downward on their outer surface. Stay away from these.
Home Depot does have some loose stock, but nowhere near as much as Ace. Lowes is somewhere in between, but the one I had nearby closed a couple of years ago.
I really can't remember the last time I ordered an OEM bolt online or at a dealer.
IIRC, the LCA bolts are 21-22mm (I just had them out recently). If I had had one that needed replacement, I wouldn't have hesitated to try my local Ace, first.
Amazon would be my next source, and in a real pinch, eBay. And then there's always the junkyard, but they're more into parts, vs. the bolts holding them in. You have to find a self-service one, which are getting rare (at least around my neck of the woods).