I got mine at the Ballard location of Fred Meyer here in Seattle last night. They didn't have any 360's in the case (in their place was a stack of $50 gift card vouchers), but I asked a salesperson and he got one from the back for me!
Head yourself over to a local Goodwill and check there; everytime I'm missing an AC adapter I can usually find one in the electronics section of one here in Seattle.