I buy deep cycle AGM batteries on Ebay from an online store called Battery-ehome (Irenehang) These batteries are brand new, but over 12 months old. They are a fraction of the new price and perform with no problems
For example: CSB AGM 65Ah for $72.60-pick up from Wetherill Park
AGM 12V 42Ah for $50.00 + $31.25 postage