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