I fear you're somewhat limited.
Remington Ultra Magnum & Short Ultra Magnum cartridges are based on the .404 Jeffery case. As are the Winchester Short Magnum and Super Short Magnum cartridges. Dakota Arms also uses the .404 Jeffery as a 'base cartridge' for their proprietary calibers (excpt for their .450). Nosler uses the .404 Jeffery as a starting point for some of its proprietary calibers.
Easy commercial loaded ammo availability will be a problem with all of these.