7.62x54R might not be popular with hunters, but it is fairly popular among shooters and collecters. Boxer primed brass is readily available from Norma (about $75/100), Lapua (about $35/100), and Graff & Sons, (about $25/100). Winchester imports S&B ammunition and markets it, also S&B is available directly at a lower price. S&B isn't the best brass, but it is usable. Haven't tried the Graff & Sons brass yet, I have been buying Lapua. Lapua brass is just about the best brass avilable in ANY caliber.
Check with Midway USA, Graff & Sons, and Midsouth Shooters supply, you can find what you need at one of them.