Archrms,
I bought mine from a local RV parts store, but they have an online store. See the link below:
http://www.pplmotorhomes.com/parts/r...20And%20Cutoff
Scroll down the page a little and you will see the Battery Brain. Make sure you choose the Type III. It is also referred to as "Gold" on some websites. Their sale price is as good or better than any other place I looked.
The installlation is easy. It even has a terminal that allows you to keep certain components (such as a radio) connected even when you disconnect the battery and also can be wired so that it will not disconnect when the ignition switch is on. (I didn't want to have to worry about the Battery Brain shutting down my car while I was on the road. I have other components that seem more than capable of doing that already!
).