The battery is lead acid, not the newer NIMH. They are still available by "special order" from Halcyon, probably till existing stock runs out. Someone probably got a great deal from an friendly authorized dealer clearing out their stockroom of old stock that they can't sell anymore and is looking to "flip" it....
There is another one that is "used" up also on eBay, they show up every few weeks from what I've seen.
It's not a bad price at all for the 18w HID lamp head if you have a 3rd party cannister you want to hook it up to, but not worth the effort if you are considering getting it to upgrade the Explorer cannister for a Halcyon cannister like the Helios 9 cannister.