>>11748673According to the bloke at one of my local Game stores every city has at least one Game store that has the cartridge, it will be in the Game store that has always been game and not the one that used to be called Game Station that will have re-branded by now.
To get it you go into the store (most shops will have it running on one of the 3DS consoles they have on display so you probably won't have to ask at the counter) start up your copy of Black or white 1 or 2 and select mystery gift, then via wireless. This should give you the mystery gift event that will trigger next time you go into a pokemon centre where you talk to someone and they hand you a Meloetta.
If you are smart you will also have brought along your other copies of Black/White 1 & 2 and you'll be able to get a Meloetta for each copy of the game you have, resulting in 4 meloettas for most of us.