MrSquarepants, Well, I am actually not 100% sure they are exclusive to Pokemon Centers. But I haven't found any other store that gets them. It might be worthwhile to ask around on eBay but their boxes are overpriced.
Glad you enjoy the card! I also pulled the X Mega UR and then a Kangaskhan UR out of my last box. I still have two boxes left that a customer backed out on, but I have enough singles to sell for awhile. Honestly, I don't know what I'll do with the stack of a hundred Rares.