A buddy of mine wants to do a very large mural on the side of a building, he's got the funding but he needs a very large wall to paint/stick it onto. When I say big, I'm talking as big as possible, this kind of size: https://www.pinterest.co.uk/pin/360358407662413145/

Obviously large walls without windows are in short supply. Specifically the aspect needs to be about 2 wide to 1 tall, and the wall must be viewable unobstructed, preferably in London or large city.

How would one go about finding such a wall?