I use two thermometers in bread baking.
I use an oven thermometer every now and then to make sure my oven is heating properly. (An oven that is not heating properly may mean you need to repair a faulty part or two; not to mention it may result in an excessive consumption of energy.) I have two oven thermometers, and they provide a check against each other. I don’t remember where I got them, but they’re both pretty old, and don’t seem to wear out or break, although the glass on the front can get dark and make the thing hard to read. Pick one up in your local grocery or hardware store for $7 or so.
My second thermometer is the one I use the most. It’s an instant read (well, almost) little probe tool. Stick it in the bread and you can tell what the internal temperature is. Stick it in milk in a pot on the stove and you can tell what temperature the milk is. Stick it in water and you can tell how hot the water is. Really valuable and cheap, to boot. They cost about $10.
There is another thermometer that I have considered buying from time to time: An infrared surface, non-contact thermometer. Whew! That’s a mouthful! This little jewel operates on the theory that a heat source gives off infrared radiation which the instrument can measure. You just point it at the surface to be measured and press the button. The instrument will then tell you what the surface temperature is. Obviously, you can’t use this one to tell you when a bread is done, but for oven heat, it might be a good deal. However, they are expensive, usually over $50, and I have better things to do with my money, but it’s something I may buy one of these days.
Thermometers: Every baker should have at least two.