The best aquarium heater I've found for this purpose is the one Walmart sells. The Tetra 30-60 aquarium heater. To adjust it, simply pull the knob off, re-position it, and keep cranking it to raise the temperature, I've gotten it to hold the temp at 101.8(F). It cost $15.

When my finances improve, I want to find a PID controller with a controlled outlet and let that control the temperature instead of the internal heater thermostat.