Flat mount adapter...
 

My local stoner bike shop gave me a good deal on a set of TRP Spyre brakes that had been sitting on his shelf a while. The owner handed me an adapter, the one in this picture, but it was used and had no hardware. I didn't realize that I needed hardware until I got home. This adapter, if I bought it new, would come with 4 bolts. Any idea if I can just get them from the hardware store?
https://cimg7.ibsrv.net/gimg/bikefor...1b63ce8ccf.jpg

Home Depot hardware isle in the drawers, plenty of metric hex head and cap screw hardware to choose from.

You may not find a match for the lower two shoulder screws in the photo. The shoulder below the cap may be important--I'm not sure. If needed, you can buy the whole set here.

Yeah. I can't tell if they're specialized nuts or not from the pics. The manual has the specs. Guess I'll go to Ace Hardware. Can anyone chime in on whether or not these are special in some way?

https://cimg8.ibsrv.net/gimg/bikefor...9de8d93bc5.png

Amazingly my local ACE doesn't have either of these in stock.

The instruction manual (from which the picture above was taken): https://trpcycling.com/wp-content/up...v-B-copy-1.pdf

Plenty of bolt vendors on the web and on ebay. You may wish to spend a little extra and get SAE (or better) quality bolts. Some of what you buy on eBay are from the far East and have the tensile strength of Manchego cheese.

Or, you could contact trp and ask if they can sell you the bolts. The whole adapter kit is $12.99, so it may save you time, energy, and worry (about getting adequate quality bolts) to just buy the whole thing.

https://trpcycling.com/product/ff-5-...160mm-adapter/

BTW, you no doubt noticed that you need different sizes for front or rear mounting.

This is looking like the only solution (buying the whole kit). I was all geared up to finish the build tonight and ride the bike tomorrow. I guess I have to do some more waiting.

Maybe that's why Home Depot was recommended?

Been in this exact position myself. But probably better to get the brakes correct before you ride! Good luck.

Exactly, and Ace's hardware bins are more expensive, I buy all my loose metric hardware at HD.

That's weird. It was my experience that Home Depot had was less in the way of loose hardware. I always go to Ace for loose hardware and HD for everything else

Nope... Home Depot skips these sizes as well...
https://cimg3.ibsrv.net/gimg/bikefor...3939d6e258.png

It appears you live in a populated enough area that specialty fastener stores should be common.
You can always shorten a longer bolt.

Here is a cap screw M5 x 14mm @ .8 (less the washer)
https://www.homedepot.com/p/Everbilt...+screw%7D%3Aqu

and the flat head version M5 x14mm @ .8
https://www.homedepot.com/p/Everbilt...3878/204841937

The 14mm may be fine but if too long just grind off the 1 mm