I found my heater (s) - (bought 2 on sale, knowing that one would break some day, and it has) in a PetsMart, or whatever, big box store that had a big tropical fish section.
There is a move to submerged 'brick' heaters that self regulated and are useless for 100F operation.
Another option I have used is surplus industrial PID type temperature controllers that came out of an injection molding machine that was getting a brains upgrade. Changed a jumper to ley it sense 20-120F, rather than 200-1200F, and ran the right thermocouple to dangle in the water bath, or have it taped to the outside wall when I used a stainless food service tray as a shallower warming dish. That tray was heated by sitting it on heating blanket, with the blanket controller set to high, and a relay driven by the PID controller. Since then I have heard of pig heater pads used for rearing young pigs, that may work better, and would be happier if they got wet.
I also have used old griddles, and food keep warm trays under stainless dishes or oven rasting pans borrowed until Thanksgivig comes around, and controlled them (not as precisely) with incandescant light dimmers.
After a while the PID controllers went flaky, and would lock up. Changing the electrolytic caps in them helped for a while, but I guess living in hot machinery accelerated the aging of some other components. In the end the fish tank heater bi-metallic is precise enough for the 12L of water and 4 1L bottles that I warm with it, and the PID controllers and thier thermocouples got tossed.