A known source for thin polarizing sheet is Edmund Optics, which sells visible linear polarizing film.

That said, rear-mounted polarizing film may not work as well as you hope on a fisheye. One answer notes that the polarization effect you want can be reduced after light passes through the lens elements, so a rear polarizer can be much weaker than a front-mounted one.

Also, the extra quarter-wave layer used to make a circular polarizer is not likely the main reason the sheet is too thick. Most of the thickness usually comes from the substrate and protective layers, not the wave plate itself.

So the practical takeaway is:

• look for thin linear polarizing film rather than circular film if rear-slot thickness is critical;
• Edmund Optics is one place to check;
• expect more flare/haze than with coated glass filters, since film typically lacks the coatings of good front filters;
• ND gels/films do exist in sheet form, but quality and fit will depend on the specific material and holder.