J.B.
If this is for your 7 year old, steer him towards something else from Lego - if he "finally" gave you an idea, then it's not something he's been obsessing about and one cool Lego set will be as good as another. If he believes in Santa, tell him that Santa doesn't make these and that Lego is sold out. If he doesn't, just tell him that Lego is sold out. We had a similar issue at that age a few years ago with a Star Wars Lego set and because he believed in Santa, I told him that he'll get whatever Santa gives him and that if it's not the one he wanted, I'd buy it for his birthday in March. I subbed in another set and he was delighted with that. Don't go crazy for the *one* item on everyone's list. It's odd how there's no rhyme or reason to this...two years ago it was impossible to get an Imaginext Bat Cave before Christmas but there were plenty be had in mid-January...