Ebay is the go. Search for mufflers in the bike section and your bound to find one thats a good price that will do the job on your bike. Ive got a GPz550 motor here to use to build a buggy. I bought a Honda muffler off something on eBay, cost me $31 delivered, its alloy about 2 foot long maybe 6" round and at the front its got a setup where a 3 bolt flange goes on it. Easy to adapt looks OK and should be plenty quiet. Im not too concerend about the sound, more keeping it acceptable for Neighbours and the whole spark arresting factor. Bob.
