Amazon.com: Replacement for APC Back-UPS CS 500 BK500 Battery - Replacement UB1290 Universal Sealed Lead Acid Battery (12V, 9Ah, 9000mAh, F1 Terminal, AGM, SLA) - Includes Two F1 to F2 Terminal Adapters :
Used APC Back-UPS CS 500 Tower UPS BK500LK No Battery 731304021360 | eBay
Powery GEL battery for UPS APC Back-UPS CS500 9Ah 12V, 12V, 9Ah/108Wh, Lead-Acid battery, rechargeable, 151mm x 65mm x 94mm, Original. New - AliExpress
Amazon.com: Hi-Capacity Equivalent of APC BACKUPS CS500 Battery : Health & Household