If you're strapped for cash, this cheap hydrofarm will work. If not, I'd get a herpstat from spyder robotics. If you get a better model, you'll have several probes, each with their own independent thermostat, all in one unit. Pretty much overkill for one tub, or even one rack (of the same species).
The basic setup goes like this:
substrate
thermometer probe
enclosure bottom
thermostat probe
heat source
Edit: Also, there is a sticky either in general balls or in ball husbandry about glass tank setups. It has a ton of info on it.
Yeah, pet shops are not to be trusted. Listen to breeders and fellow enthusiasts only. Even vets have been known to make big mistakes. Like so bad that one blinded someone's ball trying to remove unshed eye caps.