You can go to wal-mart and get an acurite digital thermometer. they're around $11 now. It has a humidity gauge on it, as well as a probe so you can measure hot side and cool side temps.

However, this will not shut off your heat lamp if things get too hot. For that, you'll need to get a thermostat which measures temps and sends power to the heat source as your temps drop.

check out Matts list of thermostats:

http://mgreptiles.com/thermomain.html