Try http://www.chimacumtack.com/ they do make them, good quality, service and prices. I have purchased donkey and mule harnesses from them I was always pleased, Amish made.
I agree with Rod. I have gotten a harness for my mule from them and they were great to work with. I bought my mini team harness from an amish dealer. It is good, but the quality in chimacumtack is better. My team one is hames and collars. Let me see if I can find their name. They were at HPD and at the 2010 NEAPFD.