If you want a really accurate thermometer, don't get one from
Walmart. Anything you get from there will need to be calibrated as well, and if you're going to do that, you might as well just calibrate the one you've already got. If you must buy another, get one of the simple, under the tongue jobbies. I used a candy thermometer for a long time, and had great hatches with it.
If you're looking for something very accurate, go for a Brisnea Spot Check. I don't have one myself (yet), but other BYC members swear by them. You'd have to order one online, which would mean a wait, and they're not real cheap either (a little more than $20 if I'm remembering correctly).
Calibrating your thermometers is super easy. Make some slush out of ice and water in a Ziplock baggie. Pop your thermometer inside. It should go down to 32F. If it doesn't, that's how much your thermometer is off. You'll just have to keep the difference in mind while adjusting/checking your temps.