Forget Winchester, Remington or S&B ammunition.
If you cannot get hold of any original MkVII ammunition (HXP etc) then the Modern commercial by Prvi Partisan (PPU) is about the best 303 in the market and performs fairly closely to the original MkVII and is manufactured with mil-spec cases.
I've posted these before, but see a comparison of both Rim Thickness and case wall thickness of Win & PPU cases - guess which is which :