I just happened on this site while searching for RSST leak fixes. Thank you for the info and pics. In regards to you asking about a supplier that sells the post insulators by themselves, Kidney Puncher sells a pack of ten for $1.95. http://www.kidneypuncher.com/aga-series-positive-post-insulators-10-pack .
If I may make one suggestion, others have stated that placing a washer between the spring and insulator seems to provide uniform pressure on the seal.