https://www.heatholders.com/
I've been using these as my sleep socks/back-up, and surprisingly, they work. My wife bought them for winter fishing(wading in 34 degree rivers for hours on end) and they worked so well compared to merino wool blends, I started to use them for backpacking. I question if they'd survive putting lots of miles on them, but I did use them a few weeks ago to hike out 11 miles , as my darn new boots are still breaking in and I had a hot spot developing. They're very soft and warm. 3.7 ozs. on my scale.