Try
www.buffaloarms.com - If they don't have what you would want, they say they will soon . . .
The first 400 rounds of 45-70 I bought (long time ago) cost me $33 per hundred. Last 100 empty cases I got for my 45-90 were $99 per hundred, and that was almost 3 years ago.
Ya got to grab them when you can and close your eyes to the price. If you know what caliber you want, get the brass before you get the rifle.
(Not BPCR related, but about 8 years ago I decided I wanted a 204 Ruger - I scrounged for primers, brass, powder and bullets for nearly a year -
then bought a rifle.)