The Nederlands and Australia are the best countries to look at when searching for a steel boat.
Where are you located? Is there a specific place where you would like to buy the boat and where do you intend taking it?
The designers that come to mind are :-
Van de Stadt
I know of a couple of excellent Steel boats that may shortly come up for sail.
P.S. Try Yachtworld USA or Yachtworld Australia - they will have a good coverage.
Also Apollo Duck HERE