BerettaUSA.com, Shotgun News, GunsAmerica.com....
I purchased 02 spare 17rd Beretta factory magazines from a Dallas TX based outfit in 2009 for my PX4 9x19mm. They were $25.00 each.
Beretta charged $35-45.00.
Check the Shotgun News ads, the main Beretta website; www.berettausa.com www.jgsales.com www.midwayusa.com www.natchezss.com
Factory new PX4 17rd magazines may be rare or cost a lot of $$$ due to the gun blitz 2013 but you may find a deal.
I do all my own stunts.