I recently picked up a PC4. I believe this is the only hi cap, 20 round, mag available. http://www.sportsmansguide.com/product/index/20-rd-ruger-p-series-40-sampw-blued?a=339239
I am considering buying one
I did lots of searching and they were all I could find. At one time, there was a gent offering a conversion that would allow the PC to accept Glock mags. How sweet that would be. Good luck with yours.