You should be able to get the 13 round mags through any surplus dealer like Cheaper than Dirt, etc. These magazines have been produced FN licensees since 1935. Canada, Argentina and South Africa all produced HiPowers. Yugoslavia made both a single and a double action variant that used the same mag.
But, I have to say this. I like using the Browning factory 10 rounders as the first mag, because it will absolutely push itself out of the magazine well. A magazine without the "mousetrap spring" on the base won't fall freely from a HiPower. This is due to the pressure placed on it by the magazine disconnect safety cam (unless the cam has been removed ;-), BTW, do you plan to use these pistols defensively or strictly for competition? ). Use the 13, or even a 20, round mag as your last magazine should you need to reload.
Semper Fi
Dave