For what it's worth, I called ZTE and asked to buy another battery, they said no to call Boost. The Boost store does not have them, and I called customer care. They were very helpful, and tried to find one, but could not. They suggested that I contact their customer service care email, which they gave me the address of
boostmobilecustomerassistance@boostmobile.com which is a damn long email, but it has not bounced....
I will update with whatever response I get.