Unfortunately, the PPS mags are quite expensive. List price is $57, and that is what they cost if you order them direct from Walther's online store that Smith and Wesson operates:
The best magazine dealer that I have been able to find who keeps these mags in stock is a company called Andean Inc. They have first rate customer service, email you a detailed invoice, and send you an alert when your order ships, including the tracking # for it. So I can highly recommend them based on the service that I have received from them.
They charge $46 for the mags. While that still sounds like a lot, I've not found them anywhere else for less money.