WHERE MEW AND LAPRAS CAN BE FOUND:
First, you’ll need to restart your game.
If you want Mew:
If you want Lapras:
Then, go in front of the Gambler. You must approach him from in front of the house, then FLY quickly before he sees you.
Then go and battle the boy (if you want Mew) or the girl (if you want Lapras)
Make sure he (or she) walks up to you, if you go too close to him (or her), the game will freeze.
Then FLY to Lavender Town.
Press B when the menu pops up.
Then Mew (or Lapras) will appear.