I don't know ion any of these would work for you since they're minie balls, but one of them might do the trick.

https://www.trackofthewolf.com/parts/detail/2724/1/ball-57-minie

https://www.dixiegunworks.com/index/page/product/product_id/1288/category/336/category_chain/578,349,336/product_name/BU0901+Dixie+Cast+Minie+Bullet+.577

https://www.buffaloarms.com/577-620-grain-minie-bullets-pedusa523577.html

https://www.ssfirearms.com/proddetail.asp?prod=HCBRA577460