Covert Mission - For hi-cap mags go to the classified section at the www.uspsa.org
web page. Ocasionally you will find people selling them or sometimes people getting out of the sport and selling all their stuff.
You can be very competitive with a single stack 45 in IPSC, but you will need a few 10 round mags. As long as there is movement between shooting positions it isn't a problem. You just have to load EVERY time you move. In fact this is a great way to start IPSC because you will get good at reloads. If a course of fire isn't 8 round neutral complain to the match director.
Shooting minor is only a handicap if you don't shoot A's. But, generally this is not realistic, and yes, it is a handicap.