Some websites sell them, maybe http/www.bigalsonline.com will have them. Or if you can't find one you can get a 25 watt heater like paul said and monitor the temp and see what you will have to set it on for it to heat it to 78 or 76.
I have These in some of my 21/2 gallon tanks they are keeping them around the 77deg and in the rest i have 25w heaters.i honestly cant say one does a better job than the other