Quote Originally Posted by Cat333Pokemon View Post
Actually, there is another way to get a Mew, but it's only for Diamond/Pearl/Platinum, and it was intentional, unlike the aforementioned glitch.

You need to own My Pokémon Ranch. Upload 999 Pokémon, then the ranch proprietor, Hayley, will have a Mew. If you bring her an egg (doesn't matter what's in it), she will offer to trade her Mew for the egg. You can then withdraw the Pokémon for usage in your game.

As for the glitch above, I can certainly vouch that it works. I have one Mew from Pokémon Blue currently in Pokémon Silver.
There are also ways to get mew from events in the US and Japan that are on GBA games. I have a few different mews.

To get the 999 pokemon for ranch you should either buy an action replay or a ds flash cart. Then download a program called pokesav and you can make any pokemon you want with any stats. You can also use a code that lets you clone pokemon.