I've seen the PTFE tube for sale at Fry's electronics in the section with shrink tube and split conduit. It came in a bag with a veriety of sizes, probably 6-12" lengths from what I remember. That was a year or more ago, so I don't know if they still carry it.
Van's includes some tube with the baffle kit to put over the stainless rod. I don't think it is PTFE. Might want to see what material that is.