Do you mean really expensive? Otherwise a cheap digital one (electronic not stick on) will do just fine. I like these: http://www.amazon.com/gp/aw/d/B00U0N0TP8 - they come in 1, 3, or 6 pack. The more in a pack, the cheaper they are.
I really do not think any of the "cheap" LFS thermometers are really accurate. Some may even be off by as much as 3-5 degrees. If you have a few tanks just get a few and get an average temp from them.
Ok, the reason I was asking is that in my 75g, the cardinal tetras have come down with Ich or Ick. I have raised the temperature to about 83 and I have used Jungle Ick Guard. I have been using it since last Thursday, and doing 30% water changes every two days. I have one thermometer at one end of the tank and one at the other end. I have a heater at both ends. The thermometers are close , one reading 81 and one reading 83, but they are both cheap lfs thermometers and I am not confident that they are accurate. Also, I am still seeing some ich spots on the cardinals. I didn't want to raise the temp much higher than 83 because of the plants. It is a heavily planted tank. The other fish, angels and rainbows are not showing any signs of the problem. So, do I keep using the ich guard, and where do I get a thermometer that is accurate?
No 1 kind of thermometer is all that accurate. If you are really concerned about a degree or two, you have more issues than temp IME. Just what are you expecting? Ich is best fought at much higher temps..~+89F at best?...but not all forms, anymore (plus salt /or other?), higher temps bring issues to some fish, but not cardinals for the most part...they're quite happy at discus temps..~88-90F..at least short term.
Have you thought of dyes or other "old school" TXs? Your plants are likely toast in an all out ich fight in whatever form...isolate them for a long time or buy new, it's a part of the war on ich if needed. Ich is not a big deal in an otherwise ideal QT environment. This is 1 of the reasons us "old-timers" use a QT tank, just something to think about next time....Sorry you're having issues.