If you don't find any in the RVing groups you might find them at any of the 3,000 or so RV Salvage yards around the US. Here's a link:
Used RV Parts
I don't know about the majority of RV'ers but what I did was buy a roll of bright LEDs with sticky backs, rated for 12 volt. When a tube burns out, I just replace both in my dual tube fixtures and install the LED strip lighting in it's place. The strip lighting can be cut to length. So in my case I keep the fixture. Lowers the cost quite a bit as I'm not buying the fixture too. Maybe that has something to do with having trouble finding people that have extras?
If you RV off the beaten track, medium sized towns and whatnot, you might find some at a really reasonable price at boat shops or RV parts stores because people aren't buying the florescent type anymore in favor of LEDs so there's not much of a market anymore.