Note:you must use a gameboy advance to use this cheat/glitch.

as soon as you start battling your rival for the first time, start typing in r,l,r,l as many times and as fast as you can. continue doing this until your rival stops talking and leaves. after you are able to set off on your own, press start, scroll down to your name, press A, and you should see over $3500 in your bank account that wasn't there before. i've tried this on blue as well, and it still worked! with this ammount of money, you will be able to purchase as many as 20 pokeballs from the pokemart. good luck!