I find the clear days are the worst. The heat just rises away and we're left with cold down here.
I got mine from the Honda dealer I deal with. They usually have them on the wall. I think the brand was Kimpex, and they come with a big god-awful looking switch, and no relay, but I can detail the info for you to add the relay ( so that you can't leave them on by accident and drain the battery, or have someone turn them on when they see the switch) and for a fuse adapter.
All in all, my grips were about $24 for the grips ( Scona had a Women rider's appreciation day and 20% off, so expect them to be around $30)
An 87a relay from Princess Auto $2
A bit of red and black wire $I think I paid $5 for both rolls
A Fuse adapter. $2
And a different 3way switch if you want.