I have a .450 Bushmaster.
All the ammo I have for it is Hornady; I have dies, I just have yet to load any for the thing. I have never seen any Remington ammo in the caliber... while it's theoretically made, I've just never seen it in stock anywhere at any time.
IMO, though, of the thumper calibers, the .450 is likely to be the one with the best ammo availability of the three. Hornady is usually in stock at online vendors and some brick and mortar shops. The .458 SOCOM has the most choices in bullets if you reload, but finding brass can be a challenge. The .50 Beowulf has just seemed to be a combination of the factory ammo selection of the .458 and the reloading choices of the .450.