Stickermule[0] and RedBubble[1] come into mind.

[0] https://www.stickermule.com/ [1] http://www.redbubble.com/

And http://www.unixstickers.com/

I don't recommend Redbubble, I just ordered some and they're cheap but the printing quality is just not very good.

I have a few from Unixstickers though, they're printed properly.

(no affiliation, just a customer)

Some sell their own, like Github. http://github.myshopify.com/products/octodex-sticker-packs