http://chucker54.stores.yahoo.net/ma727904518s.html Mancini (amoungst other places I am sure..) has the seal..

I believe the make a special puller to get it out, but you might be able to use something else, then a socket to install the new one.

(assuming that is the seal..it is under where the shift linkage and kickdown linkage all connect to the valve body..)