Kings 12V 100Ah Deep Cycle Lithium LiFePO4 Battery $249 + Delivery ($0 C&C/In-Store) @ 4WD Supa Centre

• Safe Lithium Iron Phosphate (LiFePO4) chemistry
• Light weight! 14.5kg for 100Ah capacity
• 2000+ Cycles at 100% depth of discharge
• Integrated High Quality Battery Management System (BMS)
• Thermal, Overload, high/low voltage cutoff & Short Circuit Protection

2 Year Warranty. Not the cheapest when compared to some no name eBay brands/specials. But at least you have a physical store to go to if you have issues.

Cheapest I have seen whilst keeping an eye on their prices.