I've seen them regularly in the fields of several local hunts like Warrenton, Blue Ridge, Old Dominion, Fairfax etc.
I know that Old Habit has had some. Middleburg Tack Exchange gets them in too.
Love the look of frocks....someday
Our MFH and former president of the MFHA, Daphne Wood, wears one all the time....you can't get any more proper than her! Many in our field wear them as well.
I don't know how proper they are, but some of my friends have gotten them from a dressage catalog and changed the buttons...