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I expect standard 8x57 brass will do fine, IF your dies are right.
Size a case without expander ball and see if it will take a .318" bullet with normal bullet pull. If it does, great, you are lucky.
If it is loose, you need a real "J" sizing die, not a crimp die. You cannot crimp a loose bullet tight. Most reloading die makers are good about furnishing odd sizes like that at reasonable cost.
If it is too tight, you need a proper "J" expander ball something under .318" diameter. You can likely get one the right size from your die's maker or reduce the one you have.
US 8mm is loaded lightly, it is all descended from the 8mm Remington Special of the post WW I era, which had a bullet large enough for decent accuracy in a .323" but soft enough and lightly loaded enough to be safe in a .318" barrel. SAAMI load data is similar, with a maximum chamber pressure of 40,000 CUP. Due to the larger bore cross-sectional area you can still get velocity in the .300 Savage to .308 category, which ought to be fine for hunting.
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