It wasn't an "RF Converter" back then. It was a simple switch and it wasn't even automatic.
Anyway, I remember seeing Gemini-brand clone "game switches" at convenience stores, dollar stores, and even department stores through the mid-'90s, so they should still be around. Hell, they looked identical to the originals so I wouldn't doubt it if they were the OEM supplier.
That said, Ichinisan and I first connected the Atari 2600 we found in the closet at 6 or 7 years old (late '80s) using twist-ties that were stripped and twisted together.

We had no idea what we were doing, but we made it work on an ancient black and white Goldstar (Samsung) television. That was when we first figured out UHF/VHF and all that other stuff (brute forced it!). We had no help and were literally just trying everything. When we got lucky and heard the Star Wars tune on a scrambled screen, we knew we were close and tackled it with a renewed vigor.
Fun times, even though we mostly hated our games and just wanted to play our friend's brand new Nintendo.