In a pinch in boonies, you can use 1/4x24 to replace M6x1 bolt.
Yeah, check local hardware stores, most carry metric nowadays. I actually found M7 bolts at hardware store in Morgan Hill on my way to Laguna Seca. Needed some to hold rim-halves on my BBS wheels.
You've got nearby:
Siskiyou Forest Products
Solano's Ace Hardware
Here's my go-to source for stuff:
https://www.mcmaster.com/screws/hex-.../length~16-mm/