Charlie Detroit,
Presently, I know of only one source of brass shotshells: Old western Scrounger. They are excellent CBC Brazilian cases, but, alas they are Berdan primed. CBC Berdan primers are about $3.00 per hundred, and the cases about $22.00 per 25. Most people are turned off by Berdan primers, but they are not as difficult as many imagine. The biggest problem is availability. Also, the CBC primers are not suitable for hot loads or sloppy firing pins and European brands are made from unobtanium. It is rumored that CBC 12 ga. is now Boxer primed with LR primers. If that is so, I'll have to grab me a few boxes. I have some now in .410, 32, 28 and 24 gauges with Berdan pockets. The 32 ga makes .50-70 Govt, the 24 is for my .577-.450 and .577 Snider.
There has been some Russian copper-plated steel cases and from time to time you may find some good Bell or perhaps HDS new brass in 12 ga, and the Winchester and Remington from the Korean War, oops, Police Action, or before.
Cheers from Grayest California,
Ross