Heat tape is a great way to heat individual tubs, they a slim and flexible and can be cut to you desired length then wired together so you only have one plug-in end instead of multiple. The two most common brands of heat tape are Flexwatt and THG.
I personally use and LOVE herpstats. I have three racks running off a single herpstat. They are a bit expensive but worth it in the end for your snakes safety. Herpstats have fail safes (model intro+ and up), night time and day time cycles, and uses a certain amount of power from 1%-100% to heat the heat tape to your desired temperature. Right now, in order to maintain 91F on Rack #1, the herpstat is applying 28% power to maintain that temperature.