Sepia film from the Impossible Project. It's actually Polaroid film repackaged and more expensive. Whatever. I don't know where else to get it.
Just in case there are actually any Polaroid nerds trolling this site (other than Chris) to make the Sepia film work, you need a camera that shoots black and white. Set your film speed to black and white (3000) and then move your "Lighten/Darken" knob down two notches towards "Lighter." This is because the Sepia film is 1500 ISO not 3000 like normal Black and White film.