Pretty much any powder for these big boomers is gonna be either hard to find/expensive, or both. Retumbo, rl26, rl33, h1000, vv n560, 565 and 570, and so on, it is kind of what you sign up for when you build one. I use VV N570 with 270 grain bullets.
However, Ramshot Magnum is a powder often overlooked by people that seems to do well, velocity similar to H1000, is accurate, and is what Jimmie Sloan originally used in the .338 NM's. I believe in my testing with 300 grain pills, there is still plenty of room for more powder and 250 grain pills. It is a ball powder and sensitive to temp change, but if you can't find / won't spend the money for the other good powders, that's what you have left