You need to test the temp during the boil? Isn't it generally going to be 100C, or, if you prefer the more exciting/confusing/stimulating English measurement, 212F? Wort really can't get much hotter than that before it becomes steam, what with the laws of physics and all...but maybe you mean measuring the temp up to boil, in which case I recommend welding a threaded nipple into your kettle and getting one of those dial thermometers with a metal probe that screws into the nipple.
Hmm...that was sort of racy language for a beer post...hope I don't get censored
As far as fermenter temps, I don't bother personally, but they make those little sticker type thermometers that go on the side of the carboy (the outside...I'd never risk your beer by sticking a thermometer into it.) The best way to control temps in fermentors is to control the environment they are in. Build a cool box, put a thermostat on a chest freezer...something like that. That's your best bet by far.
For mash thermometers, get a non-glass one (another threaded metal probe thermometer with a welded nipple is a great option)...you always end up stirring the mash around with it, and there's nothing so lame as breaking a mercury thermometer in the mash and going through the whole...hmm...it's *really* not salvageable?
It really isn't