I'm in the "Dallas area" and I've seen flat caps all over the place, including Target and WalMart. For a good one, you would have to go to a proper men's store, but just to try one out, why not get a budget friendly one?
depends where ya live... I got mine from a little Irish stop in town, but I imagine British stores would have them as well.
My Grandfather always wore a tweed flat cap and had quite a number of them. He only ever wore ones made by Harris Tweed. There are flat caps and there are flat caps, Harris Tweed is the latter! I'm sure you can find one in the US but 30 dollars might be optomistic.
I suggest you get a whippet on a piece of string to accompany you after you've bought a flat cap.
I purchased one from this place last year. They're in pretty good quality, but not the best.
Get a Kangol and wear it backwards.
I wear flat caps regularly during cooler weather. I have several with ear flaps folded up inside the cap that can be brought down over the ears in cold weather. I purchased most of mine a Jos. A Bank. Prices vary, but usually $30 or higher.