NBN/TPG only give you the first battery, any replacement is at your cost. Had to do mine last year, it's pretty easy to do.
After looking around for a suitable replacement I finally purchased one from my local Battery World as follows:
12V 7Ah NBN Backup Battery SKU: 12NBN7P-F2 $39.95
Everywhere else the batteries were close to or well past their use by dates, BW's were manufactured 06/20.
Hope that helps.