>>37551230it depends on how legit you want it, I got mine from an emulated emerald version where I used gameshark to teleport on the island. I then transfered it to a ruby version which was the carbon copy of my cartridge and then synced it with said cartridge via my action replay. I transfered it to diamond, then to soul silver, then to black and through bank to X and finally to omega ruby to get the trophy in my secret base.
bottom line, you certainly can find a legit emerald mew somewhere on the internet and transfer it to your (3)DS games depending on what you have at your disposal.
