I got the Slimline 4 stroke pitts muffler at Sheldons hobbies - www.sheldonshobbies.com. I live a few miles from them. Their actual store is better than their web site. The muffler is $50 which is a good price. You might want to get the 1/2" ID exhaust stack made by Aerotrend (Sheldons has it, part # AER1025) to help keep the oil off the aircraft. Attached them to the muffler tubes with cable ties. I can attach a picture of my setup if you like.

Have fun!