I used one of these to control the temperature of the Peltier cold side
http://www.tetech.com/Temperature-Co.../TC-48-20.html
This site is also useful for info on Peltiers and how to use them.
http://www.tetech.com/FAQ-Technical-Information.html
It is pulse width modulated so the Peltier will behave as if it is being driven by a smoothly varying DC voltage. It is also a PID ie proportional integral differential controller which applies power intelligently so there is no sawtoothing that you get with a thermostat. Switching a Peltier off and on repeatedly with a thermostat will put stress on the diodes by differential expansion and contraction. A bit like breaking a steel wire by repeated bending.
You can use a set voltage DC supply but then the temperature will change with ambient making it hard to match darks.
I am using the TC-48-20 controller to keep the RH200 scope heated with five dew straps to 17C +- 0.1C so focus stays constant with ambient temperature changes and there is also no chance of condensation on the optics.
If you need more info just ask.
Bert