S&W listed the P5 mags on their dealer list as of 2006
part # WAF64502
I had bought some from them before ... and IIRC they were $64.... you can get them for that much from gunbroker though too....
However I bet right now you can get used ones from Earl's for cheaper, as I think he has some of the police trade ins
Earl's current spacial page.... (just as a option for cheaper mags if you don't need them new)
and antoher FWIW... the suggested retail on the Walther P5 mags by S&W is $76