Can't find a replacement 12v battery for my Prius.

Replacing the Gen I 12v battery: you have a couple of choices
This assumes you are ok with replacing (if necessary) the OEM battery connectors with standard connectors
(connectors [12 inch negative clampw/wire and a marine type clamp with thumbscrew for positive]
both available at AutoZone, Advance Auto, OReilley, NAPA...)
--battery type 51R (450-550 CCA) fits fairly well, but the OEM mechanical clamps (to hold it in place) won't fit
(I used a couple of pieces of wood to wedge it into place and hold it steady)
--battery type 151R (maxx Everstart from Walmart, 340CCA) fits a bit better, but is a bit pricier.
Either battery should be available for $120 or less, depending on the retail outlet.