Thin, Flat, Rigid Sheets? (cheap) I'm looking for something I can put behind ink-jet printed pictures to keep them from bending, but I don't want to increase the thickness of the final product any more than necessary.

The thinnest balsa wood works but is expensive in 8" wide sheets.  Manila folder material sort of works but I would prefer that it was more rigid than that.

80lb. card stock adds too much thickness.  

I am not adverse to reusing salvaged materials for this.

Any suggestions?
How about a very thin acrylic sheet/plexiglass. It may not be as rigid as you need (being too thin), you have to check that for yourself. Also, plexiglass is not a very cheap material, generally speaking, but the price depends on the size and the thickness of the sheet, so what you are looking for may not end up being expensive. If that doesn't work for you, I'd suggest you go and look around a place where they sell construction materials and you will likely come up with a good solution.