Traditionally people use 3/16" (is diameter the correct term for thread thickness?) carriage bolts -- length depends on what surface you're mounting them through.
You can get them at most Aces, TrueValue, and smaller hardwar stores... although they won't be annodized black and may have a raised stamp (which you could grind down and polish if you so desired -- were that anal)
You can also get them from
the real bob roberts or directly from happcontrols.com if you like paying a lot for a muffler.
You don't HAVE to use carriage bolts... I'm sure size 10 machine screws/nuts would work fine. Some people go for a "flush" look and use wood screws from below -- but i'm not a big fan of that myself... *shrug*
Good luck!
Rampy