I also recommend the above, I just got my third one in the mail yesterday. Incidentally, I only need two of them but one thing I can tell you about these little units is that if you happen to have a dumb little snake that wants to use it as a climbing rock and then knocks it completely into her water bowl, it's not gonna work the same after that, lol.
But in all seriousness, check out Amazon, although their prices fluctuate since they source from all sorts of different sellers, two out of the three I got for less than $14.
