All I found for my P16 were 16's. Four of those should get the attention of most folks.
Look in Brownell's to find what 'repair' parts you can buy. Maybe a spring, a follower, a base and a housing!
Since this gun is not really for HD - its for competition - I really need the extra rounds so I dont have to change mags during a stage. (unless required.)
I think Grams, Dawson and Arredondo all make + basepads and/or spring kits for the P-16. I have Grams pads (and apparently, followers) on my P-14 mags, and they now hold 18.
Dux, call Chuck at www.shootersconnection.com and talk to him. He has all the parts you might need, and is a super nice guy. You won't be able to beat his prices either.