There are actually several options.. The round dial-type gauges that the petstore sells are not accurate, at all.. What you need is a digital thermometer with a probe. The petstore sells different versions of these, made by various 'reptile companies', but you can get a good one at wal mart for $12. The one from wal mart is by far the most popular on this forum, and many other reptile forums that I have frequented. The brand is Accurite and its grey; you can find it near the small electrical and other thermometers in the store. It has a probe and a humidity gauge. I keep one in each tank, with the main part of the unit on the cool side of the tank and the probe under the basking spot. This way, I can keep a close watch on the temperature gradient.
There are also temp guns, which you aim at a particular spot and get an instant temp read-out. These are really nice, and you can find a decent priced one. If you have many cages, something like this can make your life way easier! You don't need to spend hundreds of dollars on one, I have seen them in the hardware store for about $60, and I am pretty sure I saw one online, somewhere, for about $25...