First question, just about any double stack 9mm mag holder will work. I got mine from http://dmeholsters.com where I also got my holster. Second question, I am not aware of any finger rest magazine extensions for the PPQ SC. However, I had an old Springfield XD magazine extension that I was able rework with a Dremel to fit the shape of the SC’s grip. The most rework was in cutting the notch in front and making sure the mag went all the way in and locked. I now have about 400 rounds through that magazine with no issues. My guess is that if you don’t have one laying around you can just order one like a Pearce Grip for the XD.
You can try a Black Arch horizontal mag pouch. I have one for double stack mags and it holds my 15 round P99 mags with or without the grip extension. It also works with my P99c 10 round mags as well as my PPS M2 mags. This is my primary mag holder because of the versatility.