The .350 Rem Mag may be on somebody's obsolete list, but Remington makes rifles for it from time to time. And I believe they still produce ammo.
However, its not a big seller, and so they don't produce ammo continuosly, only doing a production run every few years.
Brass is still available, but not commonly available. My brass guy has suppled me with several hundred virgin cases over the last couple years, but it takes him a while to find it.
As far as I know, the only people who have ever made brass or ammo is Remington. I have a friend who has one of their Model 7s in .350 and I have one of the old 600 magnums.
I did find a couple boxes of loaded ammo at a gun show last year, but passed on them as they wanted $40 per box, and I felt that was too much. I can't believe the high price quoted of $100 per box, that is rediculous. Ok, I believe it, but just cannot understand why anyone would pay it.