I’ve not used one I’m afraid but I know Makita do one.
I bought their electric blanket for my girlfriend to use after winter sea swims.
it’s only a fairly small panel that’s heated, not the whole thing. Impressive how long the battery charge lasts mind – I left it running on full for 10 hours with a 5a/h battery.
if you already have power tool batteries many of the manufacturers make a jacket now. However…. a power tool battery is quite a heavy / lumpy thing to carry about in your pocket
low tech solution though…. backpack – hot water bottle – some flasks of hot water for refills.