The Polaroid 500 holder only worked with single sheet Polaroid film which is no longer made. In fact, when I last had one in the mid 1990's, it was already a museum piece, and had long been replaced by the Polaroid 545 and later the 545i. I tried using the 500 holder with film available at the time, ruined the film, made a mess, and then bought a 545 holder. All the Fuji peel apart instant film is "pack film" (10 sheets per pack), so are absolutely not compatible with the 500 holder. If you're using a 4x5 camera and want to shoot instant film, unless you can find a cache of Polaroid film, your only options are a Fuji PA45 holder, which accepts the Fuji instant pack film in 4x5 size, or a Polaroid 405 holder which accepts the Fuji instant pack film in 3x4 size.
I know that The Impossible Project (Dead Link Removed) has resurrected many of the classic Polaroid films, but the price is high (to me), and their focus seems to be more on film for the integral cameras like the 600 series and the SX-70, so supplies of larger sizes might be sporadic at best.
