Not sure if this is relevant to your needs but you can buy a reasonably priced "mural" on www.partydelights.co.uk - and if it's to satisfy a phase someone is going through they also sell very a large cardboard cutout of a dinosaur!
Alternatively, consider getting the stickers that can easily be peeled off. It's amazing how quick they are too old or bored with something. My fella wanted The Incredibles, I was really easy to pull off the sticker 6 months later when the fad had passed!!