I was looking for a 6mm x15mm bolt and nut at my local auto parts store and they didn't have them. He told me to go to Home Depot and I said "What???". Sure nough. Accross from the fastners were some cabinets with a fair selection metric bolts nuts and washers. They also had some rubber washers, fiber washers and other screws ect. I have not spent time checking it out because I found this out late in my project. I have no idea if this is common at all Home Depots. I hope this can help someone. I wish I had known sooner.

Lowes and ACE have a better selection.

but M6x15 is a rare bird. Even Belmetric doesn't have them. I've had to sub M6x16.

It may have not been 15 length exactly that I found.

I did not know that Lowes had metric.

In our area, Home Depot is better than Lowe’s. We have both next door to each other. eBay and Amazon actually have large selections.

In my area, both Lowes and Home Depot kind of suck for metric but they sometimes surprise me by having things not found at the better suppliers in the area. In my neighborhood, Ace is the place, or a regional lumber chain that carries about the same selection as Ace. Fastenal is pretty good too but a bit out of the way.

McMaster-Carr is another source, though usually in larger quantities.

Home Debit has the little M5 screws to hold the bottom cylinder air deflectors to the head.
Here we have Home Hardware which has a better selection of metric than HD.