Jim,
Before going to the extent of replacing your seal, I would try cleaning the area between the seal & ball valve. You can use an old tooth brush for this or any stiff small brush. Per the manual here:
http://home.comcast.net/~firecapt1/T...210 manual.pdf
If you do need parts call these folks: Ardemco @ 1-800-253-0115
They are very knowledgeable and have all the parts in stock.