you can not get mew in Pokemon pearl unless you use the action replay cards or swap it over a GBA card.
yes you can only do it by getting 999 Pokemon from Pokemon pearl and after you get 999 Pokemon talk to hayley and she will give you a mew!
Mew cannot be caught on Pokémon Pearl.
You can't find an actual Mew in Pokemon pearl You'll either have to migrate mew,or use action replay
Pokemon pearl
If you get my pokemon ranch on the wii then transfer 999 pokemon onto it, hayley will give you a mew as the pokemon she brings to the ranch on that day. The pokemon ranch can also be used for an extra storage system for pokemon but only works with diamond and pearl.
You cant get a Mew without hack
Mew cannot be caught in the wild in Pokemon Diamond and Pearl. You need the action replay code.
Pokesav
cheat
You may receive Mew in Pokemon Pearl by storing 999 Pokemon in "My Pokemon Ranch" for Nintendo Wii, where Hayley will trade you one, which you can then import to your Pearl game.
If you have log cabin ranch then you can receive Mew from Hayley(only pearl/diamond)
The only way to get a mew now is only to trade with another player.Dimond or Pearl I think are the easiest to trade from because if you have My Pokemon Ranch for the wii you can just put 999 pokemon in there and Hayley will give you mew.