Ok i just didnt know what your budget was. I would reccomend a Vivarium Electronics-200 or VE-200 from Reptiles Basics (rbi.com)
I reccommend that because thats what i have but Herpstats and Helix thermostats are also very good. Im not positive what sites sell these but just google it and you find them easily. Ultimately its up to you but at the very least i would suggest getting a proportional one.