Whatever one you get, just remember the most important thing to do is to calibrate it. You could spend big bucks getting one and still have it off by several percentage points.
Mix 1/2 cup table salt and 1/4 cup water in a cup, mix it so its a wet sand consistency and place the cup in a ziploc bag. Put the hygrometer in the bag (but not in the salt mixture!) and seal the bag. Wait at least 12 hours and then read the humidity. It should read 75%.
Take what your reads and add or minus the difference to make up the 75%. For example if your hygrometer reads 71% you know that your hygrometer is 4% off. So whenever you read it you know to add 4% to get the correct humidity.
Springfield Temp/hygrometer Combo Unit, Readily Available At Most Walmart Stores For About 8 Bucks. Small And Usually 1 Of The Most Accurate Right Off The Shelf. You Can Always Calibrate Any Hygrometer If Accuracy Is Suspect