Yep, no brass seemingly available for 450-400, but lots of ammo is.
Try 275 Rigby. Yes, 7x57 is the same and can be used, but if you need 275 Rigby headstamped brass or ammo, next to impossible right now.
Yes, I realize that this is a few months old but...
Several years ago at the Arms and Antique Show I found a .450/400 double rifle which sang its siren song to me, so it had to come home with me.
Yes, brass is awful to find and really expensive if you do corner some. While attending the same show there was an older man and his wife selling oddball brass and ammunition. I spent a few minutes ogling all the potentials neatly lining his table. I didn't see any .450/400 so I asked politely if there was any possibility that he might have some stashed somewhere?
He and his wife reached under the table and struggled to lift a box up to table height. It wasn't a big box but it was full to the brim with .450/400 ammunition and some brass, very little brass really...
All I could think of was Oh Boy, look out, here it comes! The price of a nice, new car!
They conferred for a short second before knocking me off my feet with an unimaginably realistic price! Then he suggested that he would take my personal check because we lived in the same State. Yikes! I almost forgot how to sign my name!
It was all yellow boxes, mostly older, with the ammunition made by Kynoch.
But life can change quickly if you aren't paying attention. A divorce and all of the 'gifting' associated with it, demolished much of my toy chest including everything .450/400. The shop was out-of-bounds since that's how I make my living and pay for that 'gifting'.
Some day, I'll stumble into another delightful find and cherish the joy of old rifles and ammunition once again.