@ashrauth,
this studs are M8x32
and those nuts are long M8 nuts but have instead of a 13mm head a 12mm hex head.
Means you can use ANY M8 bolt to fix your muffler again. Like CROSSBOLT suggests, see your HomeDepot of your confidence or any other hardware store.
This hardware need not to be posh quality. Hardens out anyway by time.