Valve have not provided any details around sourcing the batteries. All they've said is that the controller is able to be serviced by opening via the screws and clips - ie it's not glued closed like many user-hostile companies, eg Google Stadia.
The PS3 Dualschock 3 controllers are also exactly as user-serviceable - clips and screws, no glue. So I agree with OP. Batteries that are built for user replacement to a standard format are far superior.