I'm in New Zealand and have a 2001 NHW11 Prius.
Replaced my 12v battery last year, then realised later that I didn't need to. Like most modern cars the Prius has a steady current drain when not being used (for the clock, remote locking system etc) and the 12v battery will discharge over time if the car is used infrequently or on only short trips etc. My Prius draws 35 mA from the 12v battery when not being used.
Here the replacement battery was surprisingly easy to obtain. I phoned the local auto electrical place and they said they had a battery for the Prius in stock. From memory it was just over NZ$100 (approx US$70). I took the old battery with me when I bought the replacement. Compared it with the new one and said "shucks, the terminals are the wrong orientation".
No problem - the lady behind the counter went back into the store and came out with another one with the correct terminal orientation.
It was a Yuasa battery with exactly the same dimensions as the original, so no problems fitting. Prius 12v batteries are smaller than used on most vehicles because they don't have to crank the engine - just run the lights, accessories, and engine management system. The high voltage traction battery cranks the engine via a 3 phase motor/generator and inverter.
My old battery I placed on the bench and decided to put a trickle charger on it, then discovered I'd been too hasty in replacing it, as it now holds charge OK. Guess I should have just tried charging it up, but didn't want to risk being caught at an inconvenient time with a flat battery.
Since then I've got into the habit of putting the trickle charger on the new battery (in place, in the car) every couple of weeks, as I sometimes leave the Prius unused for 2 or 3 weeks at a time.