Couple of places online: soldier city dot com, bdu dot com, not all boonies are water resistant, ripstop, etc.
I got my first one at a local place (who doesn't sell them anymore), a mil-spec boonie, I've worn the bag outta this thing at work outdoors days, nights, all seasons, for about two years now, almost exclusively except for really cold windy shifts. It's fading, but keeps the sun off the ears and neck and out of the eyes wherever the sun may be. I like it for day to day wear for the same reasons--keeps the sun out of the eyes when driving, headlights, etc.
Outdoor stores, Cabelas, REI, EMS, NRS, Campmor, have them, but they are not really boonie hats I think, just fancy waterproof hats with a 360 degree brim, and not in camo. I don't think they'd take the wear and tear and dirt a boonie could withstand.
OVC