2-5gl 'tanks' are worlds apart from 20 long and the popular 12gl/17gl/25gl tanks.
My short answer for mini tanks: none that I can find. For larger tanks, check out Cobalt Aquatics Neo-Therm line.
With mini tanks, I ended up heater-less. If your room temperature does not drop below ~64F, you can keep a lot of "tropical" fish and plants that prefer / tolerate cooler water: guppies, Endlers, dwarf corries, and fish that needs higher oxygen content like stream loaches.
For mini tanks, a lot depends on your life style: I tend to work at night and to light my small tanks at night with spiral CFL bulbs - they produce enough heat to keep the tanks heated. Use more hardscape: lava rock and pebbles retain heat longer. Another consideration is tank placement: in sunlight, next to a desktop PC or other stable and known heat sources could also work, based on your risk tolerance.
Have you considered pairing odd balls for the purpose? I find no reliable (even safe?) heaters for small tanks. That has led me to use the small digital temperature controllers for the safety they provide against overheating. Then I might look at pairing them with some of the heat pads designed for heating turtles, etc. They are low wattage ,placed under the tank or vase, leaving only the small probe in the tank to sense the temperature.
An old farmers trick for heating is using light bulbs. A wooden base to hold the small tank, vase, etc. with a bulb underneath to let heat trickle up through the tank?
As mentioned, one way to get around the heater question is to snuggle the small tanks between two large ones with heat. Like in motel rooms, you don't have to listen to the heater cycle all night if the rooms on each side of you listen the theirs!