Is 9V a strict requirement?
I've used a SP-SPSB-10 battery box for a bit over a decade now. It's been rock solid with my AT831's, but that battery box is 12V and not 9V. The batteries are a little less common than a household 9V battery, but I can still very easily walk into a grocery store or petrol station to grab one when I need it.
Meanwhile my Church Audio UBB shit the bed within a couple of months of owning it. Same as the CAFS I bought from him, they barely lasted a year with minimal use...