You should be able to find .30-40 Krag ammo at most on-line ammo retailers, and larger sporting good stores. Many, many Krags still go to the deer stands every Fall.
cornbush is right about fireforming .303 British cases for use in the Krag. I have dozens of them that I've used. Only difference is that the neck is slightly shorter than the original Krag cartridge. DO NOT
, however, attempt to shoot .303 British cartridges in your Krag rifle. They are loaded to a higher pressure than the Krag, and the bullets are .310-.312 instead of .308!
Get some once-fired (or unfired) .303 brass and run them through a .30-40 sizing die.