Are there other sources for the stop/start battery in the states, outside of the dealer?

Please don't ask why I want it.

 

I don't have auto stop-start on my car (fortunately) but I recall from a loaner Jaguar that I had which had a small battery in the trunk next to the regular battery. Perhaps you can get the model # and specs off the small battery and do an internet search. I did this for my jump starter batteries when they died and there was no listing from the manufacturer. I bought the batteries online.

 

Check out this thread over on the Pommy forum: Start Stop Auxiliary Battery