US source of thin polypropylene sheets?

I'm looking for sub-mm thickness polypropylene sheets for use in plastic origami projects - sort of like the Orikaso folding dishes and polyscene's work. I need sheets that are at least 14x14" without creases. Colored stock would be great, white is just fine - but black won't work. So far I've struck out with the local plastics fabricators, signmakers, and printers, but hope springs eternal - any suggestions would be appreciated.