I believe the confusion is caused by some suppliers who call their flounder
"Dover sole" because it sounds elegant. But there is indeed a Dover sole and
it is indeed found in the states:
http://www.thriftyfoods.com/recipes
"If you have dined in very high-end restaurants over the years you may have
seen "true" Dover sole on the menu. It is a very pricey species of sole
caught in the North Atlantic off Europe. Very small quantities of this fish
are exported to North America and most ends up on tables in fine, East Coast
eateries."
http://www.bbc.co.uk/food/back_to_basics/flatfish.shtml
"There are two species that are called Dover soles. The first is the
European or common sole, and the second is the Pacific sole. The European
species is a true sole whereas the Pacific fish is a flounder. Pacific sole
is also called U.S. Dover sole."
I've enjoyed it at a couple of those "fine East Coast eateries" including
Locke-Ober. And lemme tell you, if Locke calls cod "cod" and scrod "scrod"
they don't call flounder "Dover sole"! If you ever get a chance to try real
Dover sole, you will see the difference; I surely do when I have flounder at
home.
Historical Note: I remember having Dover sole at the Margeury in New York
City in the late '40s and early '50s, when they received shipments once a
week by <airplane> and it was on the menu only that night.
Felice