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!
you don't really need to purchase a 'winter seat' most folks use big slab of rubber, trimmed to fit, or a section of an old sleeping pad, like a section of a Z lite pad, which can also double a knee pad at your tent door
+1 for the Therm-a-Rest Z-lite Seat Pad that @REI-JohnJ mentioned. After arguing (with myself) back and forth about dropping the $40 on mine, I now take it everywhere with me. I've sat on wet tree trunks, barely dried up mudholes, frozen rocks and just about everything else. It doesn't actively heat the bottom but it does keep it from getting frozen or wet (be careful in the mudholes : ) ).
I have been using an old style Therm-a-rest inflatable pad I picked up for half price because of a blemish. I love it and has served me well, but REI does not carry the new model. There is an REI branded version https://www.rei.com/product/179042/rei-co-op-sit-pad If I did not not already have it, I would either use the Therm-a-Rest Z-lite Seat Pad that @REI-JohnJ mentioned or cut one from an old or cheap closed cell foam pad. The new Therm-a-rest and similar and lower priced items, are available from that big online retailer that begins with "A". Amazon.com : Therm-a-Rest Lite Seat Ultralight Inflatable Seat Cushion, Blue, 13 x 16 Inches : Sport...
Hi @10011083 lots of good options have been mentioned. I find that the kneeling pads they sell at home depot are great for sitting on the frozen ground. They are also only $7 and come with a fancy handle!
And you could also do a little gardening while resting! BTW, sit pads also make great back pads for day packs that don't have any or very thin, flimsy ones.
Make your own. I picked up a roll of duct insulation that was like heavy duty bubble wrap and some duct tape. Cut to size cover both sides with the tape and you’re done. I made sit pads and nalgene holders for my Cub Scout den of 6 scouts for about $20 and I still have material left.