I do a few things...
I have a range on the farm with about 30 steel plates. I keep one at 25, 50, and 100 yards painted black. 90% of the time the sun heats them up more than enough during the day that I can see them quite well with the thermal. In the event that it doesn't, I just hit them with a propane torch for a minute.
If I don't feel like hitting the farm and want a quick sight in off the back porch I just use a hand warmer taped to a cardboard box.
There is a company that makes heated steel targets as well.