+1 I bought mine through Amazon from Ducks Unlimited. Since they were for an ATV and not "motorcycle", they were only about $25. They worked well to both keep your hands warm, but also dry. I'm not sure if Ducks Unlimited still sells them, but others do and for a similar price.
Some things I found useful...
1. Use duct tape to seal off every opening into the hand grip area. The idea is to stop all flow of air inside the hand mitts. You won't get it all, but the more you can make that dead area, the warmer you'll be.
2. Fashion yourself a simple "L" shaped bracket to keep the hand mitt off your brake calipers. The air flow will tend to push them up against the brake calipers. Even if you don't have the brakes activated, it'll give you space to get your fingers in there in an emergency.
I made the "L" shaped bracket out of some scrap metal I had in the garage. The long end of the "L" had a hole drilled in it to attach the bracket using the handlebar end weight screw. The "L" went forward from there to bend out in front of the brake caliper.
Chris