Greywolf offers a large selection of paper models of post-apocalypse style cars scaled for 15mm/20mm as well as a few designed for 25mm / 28mm. Scaling the figures with a printer should not be a problem, though.
Paper Models
Palm Tree Paper Models
Nintendo Papercraft offers some slightly cartoony looking paper models of palm trees.
Tardis Paper Model
Dave’s Designs offer a free paper model of a Dr. Who Tardis.
Apartment Building Paper Model
Here’s a nice paper model of an apartment building. Its designed for Battletech, but if you scaled it up, I think it also would work well for 15mm WWII games, such as Flames of War.
Paper Medieval Buildings and Figures
One Monk offers a set of paper medieval buildings and paper medieval figures.