It depends on where you're from. Pick the appropriate one from here (always compare prices... both of them have some discounts every now and then):
PosterXXL Germany
PosterXXL United Kingdom
PosterXXL Netherlands
PosterXXL France
PosterXXL USA
PosterJack Germany
PosterJack France
PosterJack Netherlands
PosterJack Austria
PosterJack Switzerland
PosterJack United Kingdom
PosterJack USA (PosterDog)
PosterJack Canada
If you're from somewhere else, I'm sure googling for "online poster printing" (in your respective language that is) should turn up some options for you.
Download the desired poster from one of the links provided in the opening post of this thread, upload the image to the chosen shop (the posters are supposed to work at a size around 60x80cm, so that's roughly the 24x32 inch option in the US stores) and order away! The posters should still look okay at larger sizes, I guess - up to about 90x120 cm / 36x48 inches or something - anything above that might become too blurry for putting it on your wall. Of course, feel free to cover your house in it, it would look awesome from the street!