Hi @pszczelarz ,
I think, that spare batteries will never have some capacity as well as an original assembled. They are assembling batteries with more capacity than it is written. So don’t worry about batteries, there is no battery as the original assembled.
Anyway, I found [https://www.maxbhi.com/battery-for-zte-axon-7.html|one store with spare parts], which is not from the ebay or amazon (in case, it’s the best way and best price to buy on the amazon / ebay / aliexpress - trust, that Qualitty is nearly same as the second production)
If you don’t trust to the online-stores, I recommend you to ask in the local service or local dealer.
Regards,