I used tohave a white Borsalino woth a black band, so you look wherever Borsalinos are sold. Also, i would think that any decent hatter would be able to swap bands for you. BTW, I assume you are talking aboutafelt fedora, not a Panama, rigt?
Bailey is a good suggestion. Check out Meyer the Hatter in New Orleans via their website. I'd be shocked if they don't have a white fedora with black band in plenty of style, size, price and material variations.