Threadless is a online community based t-shirt company where people can submit their designs. All designs are then voted on by the Threadless community and the best ones with the highest votes are printed on t-shirts, and more recently wall decals.
There are a limited number of t-shirts and hoodies printed and when they're gone they're gone. Unless they are really popular, then there may be a reprint.
You can buy their t-shirts from their website here. If you want one of their decals you have to buy them through their UK supplier Supernice here (also worth noting that Supernice also stocks other decal by other people, some of which are fab).
Their t-shirts aren't really my style (although some of the designs are brilliant). But i love their decals. Especially this one below, which has pride of place in our lounge.
They're also great if you're a renter as they can be removed with no trace of them ever having been there. Do you like them? Do you have a decal in your house?