There are three ways to get a Mudkip.
Method nr1: Pick Mudkip as your starter
Method nr 2: Get a second Gameboy and a second game. Then simply start over on the second game (select mudkip as the starter) and trade it over to your game.
Method nr 3 CHEAT! I know, cheating is wrong, but If you don't want Mudkip as your starter, or you don't have access to a second gameboy, the look around on google for a gameboy emulator. I suggest using Visualboyadvance, as is has a built in Gameshark and Gamebreaker system build in. Then simply download the rom (again, google is your best friend here) and look about for gamesharks. Simple :D
Hope this helps.