Fastest way to get coins at the FireRed Game Corner

Buy them. 50 coins for 1000 at the counter is faster than any amount of slot play, the slots in FireRed aren't tilted in your favour the way people think. Pick up the free coins hidden on the floor of the Game Corner first, there's a handful you can grab just by walking the tiles and pressing A. If you've got an Amulet Coin on something and you're fighting trainers anyway, money to coins is the route.