look for SW99 mags....
the S&W SW99 is identical to the P99 (Walther makes the frame for S&W)..... and the mags are the same .... just that the floorplates will say SW99 instead of P99.....
if they are not listed on the Magazines page... download the catalog.... they had them for $30 a pop I think... or call the 1-800-#
if not the next best thing is to go to your local sportsmanwarehouse and look there.... other than that you just end up paying $40-$45 a mag if you want a 12 rounder .40S&W mag
also check the sticky above... and look through there you might find a supplier with a special.... or you can call Waltheramerica customer service for them