I replaced my perfectly-functioning Aux battery in my 2006 Prius (I went with an Optima). If you'd like to have this battery, and can come pick it up 15 miles SW of Sacramento, CA, please send me an email: ev at darelldd dot com I'd rather have it reused than recycled.
Indeed it is still available. If anybody is interested in what I replaced it with... it is this: Prius Optima Aux The battery if fully charged and ready for action. If the price is too steep, let me know.