3.6V Rechargeable Battery Replacement

https://www.batteryspace.com/nicd-battery-pack-3-6v-1000mah-3xaa-3-6wh-triangle.aspx

The alternative to this would be a 14500 (AA format) li-ion overhauling, which is a DIY thing (needs supply/charger conversion and cell soldering/spot welding).

^:)