Flexwatt heat tape. You can get it from Big Apple Herp (www.bigappleherp.com) or Reptile Basics Inc. (www.reptilebasics.com). I've purchased that product as well as other from both retailers and have been happy with both.
I do have to say that RBI is AWESOME in setting up the connections/custom sizing the tape to a specified order and then shipping it assembled as far as the connections being wired up and all. All it takes then is installing it on the racks under the tubs. If you do a query search of the forum topics you'll find a ton of info on setting up a rack and installing Flexwatt tape and whatnot.