The part number is shown as "ARXX075".
This is readily available through several sources, including Amazon, various battery sites, even Walmart and Lowes websites.
This is a NiCd battery. DO NOT REPLACE IT WITH A Ni-MH BATTERY!
The charging requirements are different and your charger isn't designed for it. So if you find something that's supposed to be compatible, make sure it's also a NiCd battery pack.