Masterball?

My first pokemon game EVER PLAYED BY ME, was the Leafgreen/Firered Version in my GBA. Luckily, since I have 2 GBA's, i can have all three starters, BUT, i used my Venusaur 'till it reached the E4. Too bad I wasted my masterball on a Rapidash, but now, my game was deleted somehow, so I can start the game all over again. Where else can I find a masterball? Which legendary pokemon should I catch with my masterball?