There is one way, but it requires you to have not beaten the gambler you use for the Mew trick, and a lot of trainers.
First, do the Gambler trick (be one step in front, out of his view)and then push down and start. Fly to any city and battle any trainer. Then, put a pokemon with a specific special stat (it codes for a certain pokemon, for example, the special stat 196 codes for a Bulbasaur)at the top of your party, then, find a Ditto and allow it to transform into the first pokemon. Then, defeat it as normal, and then fly to Lavender Town. Walk left, and you'll find the start menu has popped up. Press B, and the pokemon will appear...
If you want Mew, use a pokemon with special stat 27.
If you just battle the trainer, then the pokemon might not be what you want.
RAWR! Who's the Boogie Monster NOW!!! MUHAHAHA