The convex mirrors are available at many auto parts and trucker supply/truck stop stores, and maybe even places like Wal-mart.
Is this a gauge for a built-in propane tank? If so that would probably be best handled by a propane dealer/supply place, the sort that rents and sells tanks for household use and has trucks that go around and deliver propane and so on. I think often they use a magnetic coupling between the gauge dial and the float mechanism. If it's the internal mechanism that needs repair or replacement, it will of course be necessary to empty the tank first.