They make them, but they are very hard to find in the USA. Here is one example:
Ceteor SOS 6V-850CA
Maybe we need to convince Bratton's or Snyder's into importing and stocking them.
However, in the short term, aren't these things essentially a gel type battery with a built in trickle charger and some meters and other accessories? Couldn't you get the same functionality out of having a spare (albeit expensive) 6 volt gel battery and just making sure you keep it charged?