I use chemical ones called little hotties.
https://www.littlehottieswarmers.com/
They are great and last as long as i can in the winter.
Just found em on amazon [ame="http://www.amazon.co.uk/Little-Hotties-Pocket-Warmers-Winter/dp/B001G6SQMG"]Little Hotties Hand Pocket Glove Warmers Winter Season Bulk Pack - 40 Pairs: Amazon.co.uk: Sports & Outdoors@@AMEPARAM@@http://ecx.images-amazon.com/images/I/41gLJthGQHL.@@AMEPARAM@@41gLJthGQHL[/ame]
I got mine by the box from Costco, chat up someone who has a card, if there's one nearby. cant remember the price, bit cheaper than Amazon I think.
Got a few free foot warmer ones in my box too. Although you can put the hand ones in your wellies, I put double sided tape on em to keep em on my tootsies! (lived down south too long)
dave