Just the viewfinder? Usually it's packaged with the auxiliary lenses. I might have one, but it'll take some looking. If I can find it, it might come with the lenses though, because I don't need them taking up space here either.
Just so you know though, I really bought into the wide angle aux. lens for the GSN and used it A LOT for a while. Alas...too many shows with rowdy people and rowdy music took their toll. The interesting thing was the GSN seemed to come apart on the lens barrel. The only thing I could figure was the aux. lens put too much flex on the cheap pot metal of the lens barrel. If you're gonna use it a lot, plan on getting more GSNs. On the other side of things, it doesn't make the lens that much wider.
Good luck on your RF journey though, wherever it may lead you.