I’m seeing two 5000 mAh batteries for $15 on Amazon.
EBL 3.7V 26650 5000mAh Lithium Rechargeable Batteries for Flashlight Vapor, 2 Packs
https://www.amazon.com/dp/B01G1G48TU/ref=cm_sw_r_sms_c_api_xbAMBb8RC22SW
UPDATE: I'm sooooo tempted here. But I just received my Copper Prince and have the Brass Reylight AA coming. So...I...Must...Resist...
EBL 3.7V 26650 5000mAh Lithium Rechargeable Batteries for Flashlight Vapor, 2 Packs
https://www.amazon.com/dp/B01G1G48TU/ref=cm_sw_r_sms_c_api_xbAMBb8RC22SW
UPDATE: I'm sooooo tempted here. But I just received my Copper Prince and have the Brass Reylight AA coming. So...I...Must...Resist...