I design all my projects to metric dimensions. That's fine for straight up 3D printed parts like brackets and game board pieces but sometimes it makes finding fastening hardware difficult. The local "small" hardware stores - Ace and True Value - all have pretty much identical selection for metric screws and nuts. The big box stores - Lowes, Home Depot, and Menards - are no different, and you can forget finding screws in any "esoteric" (read: small or very long) sizes.
Talking with some folks at work I discovered that you can order from Grainger and pick up at their physical locations. Less than $20 later and I had a meter of M3 threaded rod, enough M3 lock nuts to effectively never run out, and more than twice as many 6mm M4 hex head screws than I needed. Hooray for Grainger!