Look for projector lenses on the 'bay. Many of the episcope or edpidiascope lenses were made to combine long focal length and wide aperture. Optical quality tends to be moderate rather than outstanding but prices can be low.

I got a nice 450mm f4.5 Wollensak triplet for $23 and passed up a fascinating 300mm f2.0 (!!) because the seller wanted a few hundred for it.