This will bring closure to my question as to the source of aluminum or brass gaskets for the TT Pro 40 muffler. They fit PERFECTLY! Once again, here is a source:
http://www.horizonhobby.com/Products...?ProdID=KSJ823

FWIW - those gaskets made from a playing card work fine, but only for one use...they become so brittle from the heat they self destruct when you remove the muffler.

Thanks again to Mike Tallman and Randy!