Tried and true cheap repti-temp 500R
http://www.ihome.com/product//11442-...ermometer.html

I prefer the herpstat proportional for a better quality thermostat.
http://spyderrobotics.com/products/herpstat.html

The thermostat probe goes under the tank taped against the UTH.
To set your temperature you place a probe for your thermometer directly on the UTH inside the enclosure.
Your looking for a 90 to 92 degree hot spot.

Shame on the pet store for selling you a UTH and not mentioning that it needs to be plugged into a thermostat.
The last Zoo-med 16 watt UTH I check went to about 120 degrees plugged directly into the wall