I have heard that from a lot of people that Mew is found in a truck and you have to do something to the truck in order to fight it. Well, I decided to try on my own. My friends told me it's for Blue and Red version but I tried on Leaf Green. First, you need a friend. Don't get cut from the captian on the S.S. Anne. Instead, get your friend to trade you a pokemon that knows cut. Now, go and beat Koga (may take a while). Once you can use Surf, go to the S.S. Anne but not aboard it, instead, Surf to the right...there is nothing on the left. Once you get far enough, you'll see a long path with a truck and some pots of flowers. One path is blocked of by the pots, and the other path just ends, but there is a hidden Lava Cookie at the bottom. Anyways, I didn't figure out what's up with the freakin' truck, but I tried a lot of different things.