The best thermostats you can buy are herpstats.
http://www.spyderrobotics.com

The cheaper option:
http://www.amazon.com/Hydrofarm-MTPR...ords=Hydrofarm

When you get your thermoSTAT, you plug your heat pad into it. You take your probe and secure it OVER the heat pad on the OUTSIDE of your tank.

Do NOT have the thermoSTAT probe inside the tank, as your snake can move it, pee on it, and will ultimately screw up your probe reading.

You'll then set the thermostat to a suitable temperature to achieve your 90 degree hotspot inside the cage.

With a lamp, you'll need a light dimmer. You can find these anywhere and they're usually only around $10. You'll plug the lamp in and adjust until you get proper temperatures.