You can find plenty of options for this, but the quick and dirty solution if you just want to control one carboy at a time, is get a used mini-fridge off of craigslist or something that will fit your carboy, and plug it into a termperature controller. Here is an example of a good one:
Johnson and I think Ranco make them. You plug the fridge/freezer into it, set the temp on the controller, and it basically unplugs the power if it is too cold, turns it back on when it gets to warm, to keep you in a +/- 2 or 3 degree range.
Now, if you need a bigger space, take the door off the minfridge and build a bigger enclosure, with it inside. But that is more alterations then I think you are looking for. So used fridge, a temp controller, set it and forget it.