For the Vandalous Skeptic (Skandalous Veptic?)

http://store-xkcd-com.myshopify.com/products/citation-needed-sticker-pack

I'm into stickers, into science, and highly appreciative of wikipedia. So "citation needed" stickers is an appealing idea. Just be careful that you don't put them on any federally owned signage, as I believe that is a felony.