There should be patterns. I have like...two of them. And even if it's not Halloween, the pattern companies have most of those patterns year-round.
Fabric type doesn't matter as long as you properly interface, with a very stiff interfacing. But if you don't want the brim to droop at all, you're going to need buckram - the kind of specialty interfacing made for hat brims. I have attempted to interface witch hat brims with the superest of heavy-duty fusible Pellon, and they still drooped (it was for a consignment order, the buyer didn't care, she thought it gave them character). Buckram is the only thing I've found to actually hold proper brim shape since it was made for that purpose.
Founder and President, Madison Area Costuming Society, a chapter of the International Costumer's Guild
Strange Land Costuming - www.strangelandcostumes.com