it was an insert in a pack of promo cards. I have it as well as all the other inserts, many in multiples. I think it came in the Legendary Bird packs, the ones being reprinted in SV, but i could be wrong on that. the other insert cards included "Catch Mew", Iron Giant, Pokemon Center NY, Pokmon Center NY Christmas, E-Reader, E-Reader Ruby and Sapphire, Ancient Mew Insert, Legendary Birds Crossword, and some others i can't remember off the top of my head..
i kept several of each of them and sometimes send them as joke extras with my trades. if they are actually worth something i would like to know so i stop giving them away, lol!