Neptune Power 12V 18AH IT Gel Battery Replaces 51814 6fm17 6-dzm-20 6-fm-18 | eBay
Mighty Max Battery 12V 18AH SLA Battery for Enduring 6FM18, 6-FM-18 Rechargeable Sealed Lead Acid 12180 Backup Power Batteries in the Device Replacement Batteries department at Lowes.com