I have a PDP pro sd and I'm looking for magazines that are longer than the stock ones that come with it. I don't know much about Walther. the factory mags are 18 round. is there a 20 round magazine that fits the fs PDP?

Walther makes a +2 magazine extension which is normally used for the 15 round magazine. I'm certain that the extension will fit the 18 round magazine but the spring which comes with the extension would be too short. Probably best to check with Walther.

I imagine other companies would offer a proper length spring for an 18 round mag with a +2 extension.