There are no new p-series battery packs being made (as far as I know) because the model was sold in such small quantities. I forget the exact number, but I'd be surprised if there were more than 1,000 out there total.
You need to have your current battery packs rebuilt with new cells. I've had this done via MTO and they did a great job. Here's their info:
https://www.mtobattery.com/collections/segway-1
Looks like the p series NiMH batteries will cost you $560.
Good luck!