:welcome:to BP.net...
In addition to what the others have said, I would also recommend the Acu-Rite. Depending on which model the store has it will either be called the Weather Station for the newer model or the older model is labeled as Indoor/Outdoor thermometer. Usually you can find them at Wal-mart in the Hardware section or a Lowe's in the outdoor section. They will cost you about $12. Also, while you are there, I would look at getting something to cover 3 sides of the tank. This will help your new buddy feel more secure because the tank won't be so open. You can use anything like construction paper, poster board, foam board, aquarium scenery, or anything like that. You can be creative. Also you might think about putting some fake vines in there, at some point, for him to move around under. Plus it will help fill in some of the vertical dead space. You can usually find these at the dollar stores or arts and crafts stores like Michael's for a few dollars and they are alot cheaper than what the pet stores sell the same thing for.
If you go with the Under Tank Heater, you really need to get a thermostat to control it. Using an un-regulated heater can cause bad burns and the cost of a thermostat will be cheaper than the cost of the vet bills for not using one. There are several brands and styles available to suit your requirements ranging in price from about $30 on up.
Good Luck with your new friend and welcome to the addiction... :gj: