Matthew's suggestion of Surplus Shed is a great one. However, they don't seem to have quite the lens needed for Tempest's camera obscura. The 4000 mm lens is a bit too long, even for infinitely distant subjects. Their L 3855 is too short at 2600mm, but would focus on nearby scenes. It is larger and faster, and would form a much brighter image. It's depth of field would be much less. It could be used to focus on a screen a short distance from the far wall. Perhaps you can find a combination of two thrift store lenses of nearly the same power, one positive and one negative, to combine for the desired focal length.