Option A: is to purchase exactly the same battery and then install it. It is straight forward. No explanations needed.

Option B (my choice): is to replace with an available Li-ion 18650 to save the time order from China and assure the capacity.

I decide to pick up 3 cells from Panasonic 3400mah NCR-18650B. Two unprotected and one protected.

WARNINGS: make sure all the battery is same voltage before replacing in the speakers. Be CAREFUL not to short circuit them. Maybe balance and equalize them.