I’ve rebuilt or restored dozens of cars over the years. Replacing almost every single fastener on some. There are many fasteners that have specific requirements and you have to be careful to meet or exceed those requirements. However, with the exception of the manifold bolts, you can use any type of fastener on the exhaust. Its not critical or particularly demanding in any way. If you have flanges/hangers you might want to disconnect, use something with corrosion protection. Otherwise, just use what you can find.
The ACE hardware stores around here have extensive supply. Even Lowes has good selection.
About 10 years ago I restored a 69 vette that I wanted close to OEM. I bought hundreds of fasteners right from a Chevy dealer parts dept. A buddy worked there and I gave him lists of part numbers from my assembly manuals. There were very few I couldn’t get. Surprisingly they were very reasonably priced too.