I tried to find copper screws/bolts for a project a couple years ago; turns out they're not common and usually rather specialized (and therefore hard to find in a good variety of sizes/types).
It's easier to find copper-plated fasteners, but even with that the variety is still limited.
You will probably have better luck plating your own screws/bolts; the process is detailed on youtube and is not as complicated as one would guess at first (one of the processes uses muriatic acid, but there are various methods).
I was planning on using the plating method but ended up not doing that particular project.
Another option I saw once is to apply adhesive copper tape to the screw heads, but depending on application/skill it can end up looking pretty ghetto.