At my local Ace Hardware. Like I said, it was $27, but I think it is worth it to have a reliable temp/hydrometer. More expensive doesn't necessarily mean more accurate, but I think price is a good indicator.
I placed 3 of the springfield's in an air tight tub with an inch of wet salt on the bottom, I wet the salt to resemble the sand on the beach, not too soggy and firm. It was really interesting to see how this works. The humidity should read 75% after being left overnight in the salted box. If yours does not measure 75% then add or subtract your reading from 75%. The temperature on the other hand, well.....I layed out 7 thermometers including the 3 springfields and they all read something different within a degree or two so you sort of have to guess and do the best you can with temperature.