Barneys sold the Lanvin flat boots in black and dark brown. They sold out early in the season though. If you call around, you might be a be able to find a random pair that has been returned.
I got my tobacco brown pair from luisaviaroma.com. Unfortunately they are completely sold out as well. I never saw the tobacco brown for sale anywhere in the US.
For some reason, several of the US boutiques/retailers who carry Lanvin (Bob Ellis, Kirna Zabete, Jeffrey) did not carry the flat boot.
Good luck with your search. If you are able to find a pair it will be well worth the legwork. They are FABULOUS boots: very unique and super comfortable. I hope you're able to find your size.