Looking at pictures of Rosco (I did a Google image search for rosco p coltrane hat), it appears that the hat design varied throughout the show's run. They all share certain characteristics: black, felt, upward curving sides, but some are noticeably more pointed at the front than others. Additionally, different designs for the band can be seen.
Is it possible to pick your favorite design, print it out and take to a hat store? Alternatively, several online stores show different hat styles, so maybe you can find the right shape there.