I was at ACE in Wisconsin rapids today looking at a second gun(ruger sr9) and we were talking about my C9 when the guy comes out with a 15roud Mag for C9's the thing was huge!!
Probably a Promag. They have a lousy reputation and I believe they can void your Hi-Point warranty on 9mm guns.
If that was actually a ProMag, it's only got the 995 carbine... but you could try and make it work on the C9 I suppose. The "grip base" part would have a gap on the C9 if inserted.