Q: We are looking to find an electrically non-conductive bolt that would also work at temperatures up to 500 F max. They will be used for an electrically live part isolation kit. Anything that will fit the bill?
A: Without any understanding of your strength requirements, and focusing solely on the temperature and conductivity requirements, one of the best materials to consider would be PEEK glass reinforced polymer fasteners with glass fibers (for extra strength). PEEK screws are usable to 500F. Also, keep in mind that PEEK glass-reinforced are non-conductive, but carbon-fiber reinforced PEEK will conduct electricity.
Vespel is also another polymer option as it gives you a bit more room on the high side of your temperature requirements, but Vespel screws are VERY expensive and have limited availability in terms of raw material.