First, try the codejunkies.com European site. Then, try at cheathappens. If not, then try supercheats.com, I'm sure that you should find something. I'll look for it, but I don't know how successful I will be.
I don't remember them off of the top of my head, but:
1) Go to codejunkies.com and go to the US site.
2) Get the Master Code for Pokemon LeafGreen.
3) Then, on that page, there should be a list of cheats. The Mew/Celebi ones might not be on there, but they are definitely on supercheats.com. If I get the chance, I will put them on this board.