I stand corrected. My local gun shop told me the design was too new and that no pre-ban mags existed. It wouldn't surprise me if they designed a new pistol around pre-ban magazines, though.
I did a thourough scan through Shotgun News and found exactly one place that carries hi-cap P99 magazines (Forrest Inc 619-561-5800). They advertise P99 15 rnd for $29.50. Personally, I don't see how it can be pre-ban at that price.
So, sorry if I gave bad info. If you do find some, buy them. As pre-ban mags go, these seem pretty rare.
LL