Brass
Ballistic Products has a brochure about loading brass Shotshells. It contains smokeless load data for use with Magtech hulls.
I am not aware of any commercially loaded brass hulls. It really is a do it yourself operation. As noted by an earlier poster, it can be done effectively with a few simple tools.
The one sorta big problem with Magtech hulls is if you want to use modern components like plastic wads and 209 primers.
Modern plastic wads are too small for drawn brass hulls which, though nominally 12 gauge, require 11 gauge wads. Priming requires large pistol primers.
If you want to use smokeless and modern components together, you need hulls from Rocky Mountain Cartridge Co. These are lathe turned and use 12 gauge components and are 209 primed. Extremely well made, very durable. They are also expensive. Last check was about six dollars per hull.
Ed Hubel sells components not loaded ammo. His focus is on slugs and has produced some remarkable and useful data.
Here is a pic of a Rocky Mt. Hull loaded with a one ounce BPI Thug Slug a la a Hubel recipe...75 grains of SR4759.
A reloading kit for Magtech hulls: two antique tools available on EBay are the tong type tool for boxer primers and the tube an plunger for setting wads and dropping shot. The depriming pin is from a Lee Loader.