bricklink might also sell sticker sheets but I'm not sure
I know they still sell the little cardboard piece from the original Hogwart's Castle, and I think they have a section for sticker sheets IIRC.
What annoys me about Bricklink is the individual shops. The one's I've tried interacting with have very..."unique" sellers, who charge ridiculous (and incorrect) exchange rates, and for certain countries (like) mine, charge an extra "tax". You also can't pay directly, but you have to contact the seller, wait for them to contact you back, then you discuss everything, and then you finally buy stuff.
I've never actually made a finished transaction on Bricklink because I cannot deal with the shotty system it runs by. I'd be happy to pay a little extra on eBay or any of the other online sellers if they can just shut up and take my money.