Probably better off in the ammo section.
Not really sure what the question is, but corrosive milsurp 7.62x54R is available from frontier taxidermy (they've got their banner at the top of the page). You of course can't hunt with that stuff though.
As far as hunting ammo goes, it's around... not rare, but some stores may not have it.
Edit: after rereading, No. 7.62x54R does not have an "equivalent" cartridge. The 308/7.62x51 thing resulted from the americans using different names, and some jumble of other reasons I've never paid attention to. 54R is 54R, fortunately though, for plinking 54R surplus is pretty cheap.