When I got my 'first' Brno 7x57R/12ga., initially I formed the brass for it from 303Br. cases, basically by running it through a 7x57 sizing die. As I recall, it comes out a touch short but it does work.
My first 'real' 7x57R brass, some Norma and some RWS, I got from a guy on site, gunbug. He seems to have an 'in'. Another guy on this site that may be able to help you out, Henry Nierychlo.