... Check out Home Depot. They carry (among other models) the Basement Watchdog. It's a battery unit. The combination pump and battery AC/DC unit is about $450. I know you only want the backup to the 'main unit', but I though I'd just mention it because I was recently looking at them.
My best to ya and hope this helps.
Jay J -Moderator
PS: HD isn't the only place to look. I don't know if Sears carrys sump pumps, let along backup units ...