Check the classifieds on here or ebay. All of the generic and cheap new mufflers I have seen, including the one I bought, are quite loud. Odds are that the cheaper the muffler, the smaller the muffler, and ergo less muffling. Small, cheap mufflers also flow a lot more and obviously that would require jetting.
Full size generic mufflers that won't get any inordinate amount of attention are going to be something like $50+ each for new and may require some work to get them to fit right. Generic, cheap mufflers also tend to have cheap muffler packing. Cheaper muffler packing makes the exhaust note louder and crappier. It is possible to buy new muffler packing. Applications specific to these bikes, whether oem or oem replica, are going to be much more expensive but probably won't require any custom fitting or jetting.
If you're open to custom fitting you can also potentially find a used oem muffler from another make and model that fits close enough.
