I know some people use the lamp dimmers (rheostats), but I would really like to suggest you go with Emily's suggestion. Even when I had only one tank I used a $70 thermostat. Heck I set up a second tank which was not identical to the first, so I got a second thermostat to control that setup. There is a bigger peace of mind that comes with those units, and are completely worth it to me. The other 'cheaper' units have a higher rate of shorting out or not holding temps well. Not to mention are more prone to temperature swings as the room temperature changes or power outages.

Most heat mats will heat up to over 110 degrees, so it's is absolutely essential that you be able to control those temps. The vet bill alone will way exceed $70 for a burn, so it's better to prevent it in the first place.

Again, if money is an issue then you may want to hold off getting a snake until you have a bit of savings in case any issues pop up.