You can find a GameBoy Color for a pretty good price. I got mine at Highscore Videogames for $60. You would need a flashcart to play ROMs that you create using this. You can always turn to emulators if you need to save your money.
I recommend the gameboy advanced ags 101 model. Has the fullest compatibility with every gameboy/color/advanced game. And the backlighting helps tremendously with gameplay.
