Why battery powered? You are sitting on a vehicle with more surplus electrical power than it knows what to do with.
I have just ridden my feeble Himalayan for close on 350 miles, with it powering heated grips and an electric jacket liner and none of that independent battery nonsense.
At its simplest, you already have an electrical supply, through the flying lead of your Optimate / BMW battery charger.
All you need then is a means to regulate the heat of the clothing. This is very easily dealt with through the straightforward use of say, a Thunderbox, which is all but ‘plug and play’. I’d wager that it would take no more than an hour to have the whole lot installed and up and running.
The fault with many battery powered gloves / liners is that the regulator of the heat is (more often than not) installed in the cuff. The cuff is then covered by the sleeve of the jacket, rendering the affair all but useless.
As regards makes? The three most popular here in the UK are: Keis, Gerbing and Warm’n’Safe. In Holland / Germany, there are additional brands too. To all of these can be added the Chinese copies (or originals with a different label) which feature on Amazon / EBay.