DIY Outlet Box-Help with wire choices

I am building an outlet extension box which will use 2 Shunyata outlets and a furutech IEC inlet, all put inside a nice aluminum box. I will be plugging this into my Shunyata Hydra4 to extend the amount of outlets for the system.

I am looking for suggestions on 12 gauge solid core wire (copper or silver) to use internally. Can someone steer me to the correct web site and offer their thoughts?