the 7x72R is one of the oldest Wilhelm Brenneke developments. in fact a 9,3x72R case necked down to 7 mm, looks like a real small bore nitro express cartridge http://old.municion.org/7mm/7x72R.htm

you can use 9,3x72R brass and modern 7 mm bullets for reloading.

the 24 ga is still available by FIOCCHI https://fiocchi.com/en/catalog/your-passion/hunting.html?animals=6362&gauge=6214

and its not expensive at all but cost similar to more common caliber. I know it because there is also a 24 ga hammergun in the safe. of course, I have 24 ga magtech brass in case it would dry up once.