I have had great luck with factory 10 round magazines in full sized guns. Those would be my first choice for the 31. I have had good luck with the Magpul 10 round Glock 19 magazines. As noted in a previous thread, I have found steel ammo and Glock Pmags have not worked well for me.
I would also keep an eye on the court system. There could be a change coming in November that would allow you to legally bring your normal capacity magazines into California.