I think this page at MidwayUSA might have what you want. One of them is .450/.400 “Thin Rim” which I didn’t see in The Handloaders Manual of Cartridge Conversions. That one is on sale and in stock, so I hope you can use it.

.450/.400 at MidwayUSA

Here it is at Graf & Sons. It’s out of stock, but something to watch. The price at Grafs includes shipping.

.450/.400 at Grafs

I don’t know much about double rifles, so I’m not sure these cases are what you need.