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. 🙂