You can buy an alternative battery for MATE X bike at this point.
One option is to search for the alternatives on AliExpress. This is an example (make sure to check for the compatibility with the supplier before purchasing).
The other option is a one-man organisation and it seems that the battery is very good, better then the original MATE X batteries, as the creator says.
As far as I understand, the design was done specially to be compatible with MATE X bike but also to contain quality parts. Also, there are couple of options to choose from:
So far there have been 3 batches of batteries produced and shipped to customers.
Communication is very open and transparent and you can check it out for yourself on Mate X battery project.