Sometimes, one can find Norma or German ammo for a modern 8X57, but be sure you know if you have the old 8X57mm J (.318 bore) or the 8X57mm JS which became standard following 1905, and which uses a .323" bullet.
There are also rimmed forms of each, meant for double rifles and "drillings" with both shotgun and rifle barrels.
There are a number of other 8mm rounds, so be very sure what you want before ordering ammo.
I suggest that you buy, "Cartridges opf the World" by Frank Barnes. He died some years ago, but it has been updated by other authors. Look at Border's or other major booksellers, where they stash, "Gun Digest" and other titles by the same publisher.
Lone Star