Does anyone know where I can find a 12v battery for a Prius tomorrow in the NYC area?

FlagMichael is correct. Any Toyota dealer will have them. While you can perhaps get by with a jumpstart kit if the battery is not totally dead, you will have to jumpstart every time you stop the car so it's a poor solution at best.

The correct answer is to prevent this situation by changing the 12V battery every five years. Because the Prius doesn't have a starter motor or an alternator, there is no warning that the 12V battery is getting weak.

I have no idea why anyone would have given FlagMichael's answer a thumbs down. The other answers were incorrect.

You can cheat by getting a jump starter and leaving that connected on the battery. The demand is low enough that the low capacity and substandard connections don't matter. When you can get a real battery you will have a jump starter, already charged and everything.

You cannot leave the battery disconnected. The 12 volt DC/DC converter (no alternator in the Prius, as you know) is not stable without the battery. Even a weak aux battery can drop fuel economy a surprising amount.

if you can get the car started then you won't need the battery, get some jumper cables and a jump start from someone and dont shut off the car until you can get another jump start.

when you are driving the car powers everything off of the alternator so if you can do that you should be fine

or try walmart :)

Any parts store and or dealership, there no different to get than any other battery,,,,,,,,,,