I'm definitely an AR fanboy. While we all had ARs, not a single one was setup the same way! Eventually, I'd like to run the courses with other weapons.
I first need to save up and get some rifle mag holders, shell holders, etc, a shotgun, etc.
It is true that Rich's 3Gun shotgun setup (that he is selling) is a good one.
I'll also note that I started with a Rem 870 12 gauge pumpgun with an 18" barrel with no choke, and an extended mag tube. Worked perfectly fine. Yes, you can shoot a semi-auto faster---but reloading speed is the main thing on shotgun. So a decent cheap shotgun works.
If you are going for a semi-auto, the Mossberg 930 JM Pro is probably the best buy out there for a multigun shotgun, out of the box.
AR-mag pouches you can get anywhere---I like the plastic belt-mounted ones, but you'll see some people wearing tac vests, too. $30 for the Blade-Tec or Comp-Tac belt pouches.
The shotgun shell holders, on the other hand, are absurdly expensive for pieces of molded plastic. [sigh] The good thing is that they don't wear out, but it is really annoying to have to spend that much money.
I used to get a picture of the gun rack during previous Multigun matches---it was always fun to look at all the different mods people have done to their guns. Fun having a rifle you can customize in so many ways!