The Retoport goes for about $20.00, you can find them on Amazon http://amzn.to/2kIoT1P.
The RetroGEN Adapter is to play Sega Genesis on the SNES now this just like the Retro port but with this one, it doesn't stick out as bad since the Genesis carts are pretty small compared to US NES carts. Just like the Retroport you have to use an RCA plug to get video signal and it only uses part of the SNES to do the work.
The RetoGen goes for about $20.00, you can find them on Amazon http://amzn.to/2kIoT1P.
The last one is the Super Retro Advance also made by RetroBit and unlike the Super boy that was made for the SNES this to also needs you to use RCA cable to get it to display on your TV. Just like the other adapters, this does a great job with colors and gameplay is very smooth. No report of display problems or certain games not working but the Super Retro Advance can only do GBA games and can not do GB or GBC games. It has a game link connector to it so you can play your friends that might still have thier GBA. Now this won't replace your Super Gameboy but now your SNES can be one heck of a retro gaming system with these great adapters.
The Super Retro Advance goes for $36 and you get them on Amazon http://amzn.to/2k6nRh3