You gotta keep watching. Winchester made a run of 375 last summer and Graf & Sons had Jamison brass in January. I don't know why there isn't a company that consistently produces ammo or brass but it must be low demand. Haha! The Winchester ammo sold out the same day it hit MidwayUSA. Also, be careful, some loaded ammo is 30-30 brass blown out to .375 and won't take the pressures. Just my 2 cents.
Did you not read my post above?
Reed's Ammunition & Research, LLC offers several different loads of 375, loaded in 375 Winchester brass. I ordered 3 different loads at a decent price and received them in less than a week. The price, IMHO is pretty good considering you get loaded ammo then have once fired brass to reload.
I haven't fired any of them yet and won't be able to until I return home in September from Europe. I will order some more loads this week to be waiting on me when I get home, because I believe, what you have you have.
I also posted a thread earlier that gave a heads up on available bullets and brass for both the 375 & 38-55. The brass was blown out 30-30 and was not all consistent in length, but I will use the ones (99 %) I saved for plinking loads.
I also ordered and received Heavy 38-55 ammo from Buffalo Bore because they can be safely fired from the Winchester 94 BB 375. Thought I'd give them a try.
My point is, there are options out there whether you want factory ammo or components to reload.