Any regular 15 round mag from the fullsize will fit the compact. Just don't slam it in if the slide is open because it could hit the ejector hard and break it.
There are some mag sleeves U can use to put over the longer mags to make them fit the compact better. I bought 1 from Germany recently. They are on Ebay. If U look, there is 1 or 2 threads on these sleeves here in the Walther section. I think I took a pic and posted it.