The monthly Pokémon newsletter for North Americans is going out with codes to get Mew. If you missed your chance to get one last February from GameStop do not miss this opportunity! Haven’t gotten an email yet? They may not all be sent out, so please be patient. Also check your spam folder to ensure you don’t miss it.
If you have a PAL game (UK, EU, AU) you can claim Mew by using the code MEW2016