After you obtain your Mosin, visit http://7.62x54r.net/
and determine the history and probably bore diameter of your Mosin or, if you prefer, slug the bore. That will tell you what diameter of bullets to use.
My Finnish M27 prefers .308. About the only commercial loads I know using .308 in 7,62x54 is Norma, or at least they used to, and were quite expensive. However, garden variety .308 150gr are relatively inexpensive to load.
If you need .311 bullets, then you can get PRVI Parizan factory loaded for $15-16 at gun shows and most online places like MidwayUSA and Grafs. Grafs also sells PRVI bullets which are a nice inexpensive jacketed option. I us them for .303 Brit and 7,65 Belgium Mauser.