I tended to make it myself. Save up your money to buy the schematic from Moira (or one of the other vendors). The NV leveled lists for ammo makes .308 virtually nonexistent at low levels, and rare at high levels, same as .50MG ammo.
One caveat - primer for large rifles tends to also be rare, so I usually make my own recipe to change small rifle primer into large rifle primer. I also make a recipe for converting small handgun primer into large handgun primer for the same reason. Buy schematics from making large caliber ammo (don't waste your money on small ammo as that tends to be plentiful), and make those recipes for primer conversion and you should be able to handle large ammo guns/rifles.