i have a cheat that lets u have any pokemon in the game
d2000000 00000000 (sorry if i screwed up in the typing)
first, you press L+R in the game. this will give you 493 masterballs in the items slot in the bag. if you were looking for a mew, you would subtract 342 masterballs. this will leave you with 151 masterballs. that is what number mew is in the national pokedex. you would press L in a patch of grass, run around in it, and you should encounter a mew.
press L+R again, and you will get the masterballs back. do this to get as many pokemon you want. this code is only for diamond and pearl