Saltfish, I frequently buy reloaded ammo (quality reloads) from a company called Georgia Arms, as they use great stuff (Speer Gold Dot, Nosler Partition in rifle rounds, etc) and they come to the major gun shows in my area. Years ago, I remember looking for .30-40 Krag for a friend of my grandfather. I wandered by their table and they had in three different loads. I was shocked and thus spent some time perusing all the old ammo they had. There was .45-100, 7mm Arasaka, .30-40 Krag, .38-40, etc. All sorts of older cartridges, loaded to original specs.
I'm not saying that they'll have 8mm Mag, but you might want to send them an e-mail and see if they have a line on the shells.
http://www.georgia-arms.com/