What RTC said,... most people go with a stock Harley bullet muffler, Sportster or Dyna,... a few have had trouble installing, (it's not a direct, perfect fit),... but, it's easy enough...
Sportster, Dyna,.. l
look for a rail mount, not a tab mount,... no crossover tube,... and not de-baffled, if you want reasonable volume level... de-baffled, might as well be straight pipe...
Rail mount,.. looks like this...