The 12 round standard capacity .40 P99 mags are Law Enforcement Only right now, but in 7 weeks, the 1994 "Assault Weapons" ban will expires, as Bill said.
Now, if you want to get a jump on things, and you have a dealer that will work with you, CDNN has SW99/P99 12 round Law Enforcement mags on sale in their latest flier - BRAND NEW for $18.99 and DEMO for $12.99. Your dealer can order them for you, hold them for you, and you can pick them up on Sept 15.
I've been thinking about doing this but just haven't got around to it yet. Even when the ban expires, the $18.99 is a pretty good price, since 10 round .40 P99 mags usually cost $25 and up