At the electrical isle at HD or Lowes, on the bottom shelf is usually large boxes of bulk items. You can get a double gang box for about a buck. Make friends with a pack rat like myself and ask him for a cord with a plug. You can also buy a cheap extension cord for about three bucks. While you are shopping, buy your dimmer, an outlet, and a double cover.
The double box will have some plastic ears with mounting nails in them. If you aren't going to mount it under the tank in a stand, I'd cut them off just for looks. Wire the plug end of the extension cord to the 'in' side of the dimmer and wire the electrical outlet to the 'out' side of the dimmer. Screw them into the box and add the cover. When you are done, just plug it in and the heat lamps and/or hot rock into the outlet.