Amazon.com: 12V 7.2Ah UPS Replacement Battery for APC ES500, ES550, LS500, RBC110, RBC2 by UPSBatteryCenter® : Health & Household
Mighty Max Battery 12V8AH for ES500 ES550 ES750G LS500 RBC110 PX12072 Rechargeable Sealed Lead Acid 1280 Backup Power Batteries in the Device Replacement Batteries department at Lowes.com