Starline has 375 Win brass in stock, if you're a reloader. So can make your own 375 Win ammo.
450 Marlin is harder though as Hornady stopped making the brass years ago and nobody else made that brass (and you can't form the brass from any other cartridge unfortunately). So unless you can get your hands on the old brass, you're limited to their factory ammo and that brass after you fire it. Which they only make the FTX ammo now, so the brass is 1/10" shorter than standard brass to accommodate their FTX bullets..