I used to get Gel.AGM, (ie,no fluid but impregnated acid fibre pads between the plates) second hand and already 4 to 6 years old, they came from mobile phone mast installations in Europe by the pallet load. I paid scrap price.
Unfortunately today the big companies and service agents are no longer allowed to dispose of so called hazardous products, and they have to use licensed specialist recyclers.
Old Hawker Powersafe are still available, but the source supply are now dubious, and the second hand price is ridiculously expensive. I now buy 12v Sealed lead acid, new, golf cart type batteries.
After 12 years I now have just 2 strings of 4 Hawker Powersafe in operation down at the 1300ah 48v battery shed that are good, and the 3 in my photograph, which have fallen to a holding voltage of 12.9v and are no longer on active duty.
Certainly the Hawker PowerSafe batteries have outlasted all my other AGM's. Not bad for 17 year old batteries in daily use.
In the photograph that grey AGM is a Marathon by Exide made in USA, its the last of those and is now just holding 12v so its about scrap.
Photograph showing my test rig for bench testing OzInverters.