Thanks for reaching out!
Unfortunately, it does not look like we have many products in stock at this time that meet your needs. First, take a look at the Ignik Heated Seat cover. This requires a power source to work, but could be a perfect addition to a seat you already have, or something like the REI Co-op Sit Pad, which is open-cell foam (air can circulate through the foam) so it will provide comfort but not much warmth by itself. While it is currently out of stock, the Therm-a-Rest Z-lite Seat Pad is a closed-cell foam (air cannot pass through) pad that can prevent your body heat from being pulled away by the cold ground. The Therm-a-Rest Z-lite, Nemo Switchback, and Exped Flex Mat Plus are all similar closed-cell foam sleeping pads that fold up pretty small and can be used as seats in a pinch. In fact, I have an older model of Z-lite pad that I cut in half that serves as my seat and/or insulation from the ground if needed. Lastly, while REI no longer carries it, Sea To Summit makes an Insulated Air Seat that appears to be available directly from their website.
Hopefully this helps, thanks!