As I'd noted elsewhere, at the shot barrel, Snider brass would work, although a bit short. Magtech 24ga brass could work too if you can get it.
Graf's has the magtech brasshttps://www.grafs.com/retail/catalog/product/productId/11079
50 Alaskan brass might work for the rifle barrel, so 348 Winchester could be used as a parent case.
Graf's has ithttps://www.grafs.com/retail/catalog/product/productId/77981
Furthermore, if you find that this gun would end up being more trouble to load for than you would be comfortable with, I'd like a chance to buy it from you.
Consider me if you want to pass it on