You can order stickers from the following places:
Diginate prints the hexagon stickers for Mozfest and also do foil stickers. They ship internationally.
Juststickers mostly ships within India. International shipping is a bit expensive unless ordered in bulk.
Sticker Giant is a great company that will print hex stickers on a roll or individually. Will ship to the U.S. and internationally.
Stickermule mostly ships to the US. International shipping is more expensive.
Redbubble offers one-off die-cut stickers.
hexagon stickers tesselate cleanly, as seen on @maxogden's laptop.
there is also an official hexagon sticker specification, the Sticker Constructor Spec