They are available, i don't know what the prices are like over there though. I got a price on one for a 763, its a spark arrestor muffler and was just under $200 AUD. The key is actually finding someone with them!
Take your old one to a muffler shop. There are a couple of muffler shops around that will build a custom muffler. I do this for my machines and I work at the dealership!