There are such things as heated socks, operated by rechargeable batteries. I haven't tried them - yet. But they get pretty good reviews on amazon.
https://www.amazon.com/GREATSSLY-Continuous-Rechargeable-Motorcycle-Arthritis/dp/B0819FVTLL/ref=sr_1_5?dchild=1&keywords=heated+socks&qid=1577057312&sr=8-5#customerReviews
Also L.L. Bean is a good source in general for all things to make you warmer.