I recommend buying the good stuff first as it is no fun shooting a match with guns and gear that don't work. The Wilson 47D mags have worked great for me. I also suggest the extended base pads.
For holsters and mag pouches check out bladetech, Comptac and Kytac for the best and most popular stuff.
As stated above Brownells is great place to buy stuff and so is Velocity shooter, shooters connection, and CPWSA.