Many people use it with powerbanks.
Most will work, but you can also get the ones tested by RICOH.
from official RICOH FAQ
How can I charge the battery? - トピックス(米国)
Connect RICOH THETA Z1 to a PC using the bundled USB cable.
Additionally, we have confirmed that RICOH THETA Z1 charges when connected via the bundled USB cable to the devices listed below. However, be aware that this use falls outside of the warranty, and the customer assumes all responsibility.
Portable battery chargers
- Anker PowerCore Speed 20000 PD
- Anker PowerCore 13400 Nintendo Switch Edition
- Anker PowerCore 10000 PD