You can't catch either in the game.
For Mew, you have two ways. One is to get the special event in Emerald, where you get a ticket, and go to an island, faraway island, where it is caught. Then transfer it by Pal Park. Or you can get it from Pokemon Ranch on the Wii, where you need to trade for it.
For Deoxys, the same thing goes about the island thing and Emerald, except that it is birth Island, and then the transfer. GameStop had a thing where they were giving them away, but this is over.
they are diffrent forms of deoxy
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
You cant get a Mew without hack
you can not get mew in Pokemon pearl unless you use the action replay cards or swap it over a GBA card.
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)
No either trade our hack