Hello Greg,
I think this is an excellent question you posed. I am unsure if you need to stage all rooms, or just one to two to get the feel of it.
I can think of a few options that might be of some assistance to you though.
I. You could always try craigslist, estate sales, or the dump to find a mattress and boxsprings, since having a used/dirty one really wouldn't matter. You could possibly get a huge discount for looking here first.
The reason I recommend CL is because mattresses and boxsprings for this purpose don't have to be functional, only to convey the look, right? If you purchased a nice bedding comforter, pillow casings and a skirt, I think that could hide any and all dirt/damage.
II. You could possibly get a deal by buying in bulk all of the beds at once in a mattress store.
Storing these beds and all the other projects you will undertake might be an issue, but if you have consecutive deals it shouldn't be, or dirty/used/old mattresses you got for cheap, I don't think you'd feel bad for tossing them out.