Alexander Startsev

Need to fabricate transition from 3" vband to stock cat (something like 2.25 i believe pipe is, plus moronic gasket, plus nonstandard cat socket).
so i need this header side flange. ideally - intire piece or just glange so i can run 3->2.25 reducer through it.
how is it called and is there a place to buy?
Or maybe there is conventional way to retrofit something there?
Thanks