The only place I know of on your side of the pond is Richard at Europa Engineering in England (Welcome to Europa Engineering). I don't know if he'll have the same cylinder or an equivalent one, but he probably knows more about Europas than anybody. He also has a lot of cool upgrade kits.
Hi Raul and welcome to the forum. I don't have a solution to your master cylinder problem, but the people at Dave Bean or RD Enterprises may have some suggestions for you. I believe the Europa master cylinders are getting hard to find, but there are alternatives in use. On my car the brake boosters have been removed and the master cylinder has been replaced with a Lotus 7 unit with a smaller bore (to compensate for the missing boosters). I think there is a Nissan master cylinder that is sometimes used as well, and I'm sure there are others. Try contacting the people at the links below for suggestions;
Is it a dual circuit master cylinder or a single circuit one ? Single circuit ones are easily available, dual circuit not so much. There are some alternatives which could be made to fit with modifications I guess, although it would also depend if you had servo assistance in the system.