Rhino Bullets, a London, England company, makes a 380-gr. bullet available from a place in Salem, Oregon:
Safari Bullets | Specifications
I don't know how it would work for longer ranges, but it's the heaviest one I could find.
Note the recoil with such a bullet using maximum loads will make accurate shooting very difficult. I know of a person who used 350-gr. custom made boattail bullets in his .375 H&H for 1000-yard matches back in the 1960's. The 14-pound rifle's recoil was brutal. But accuracy was very good only when he was able to hold the rifle repeatably from shot to shot as it moved quite a bit while the bullet was going down the barrel, just like large bore double rifles. He finally gave up on the project and went back to 30 caliber magnums.