The unit I have came from Canadian Tire, but I've also spotted it available without the 'Motomaster' branding from a company in the UK, so it may also be available in the U.S., but I don't know who would be distributing it there.
Here's a link to it, NOTE: the first one I bought died when I had an electrical glitch (ammeter burnout and battery boiling over), so I replaced it and it worked fine last year, however the reviews on the website are mixed, so try to find one locally so that you have the ability to return/exchange it if you get a dud.
Canadian Tire "Eliminator Battery Guard"
U.K. seller 'Motrax'
Manufacturer's Website in China - maybe a quick note to them can get you a list of local resellers? This site also has the install instructions so you can get an idea of what's involved (pretty simple)
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