The recommenders can be from anywhere in the world – they do not need to be from or in the United States. They also do not need to be working directly in your field. They should, however, be qualified to testify to your extraordinary ability through their work in a related field or through their in-depth knowledge of your work and career. Former or current clients, bosses, collaborators, peers, curators, friends – all of these are excellent recommenders. It is generally a good idea to restrict the number of teachers or professors to 1 or 2, though; the government is more interested in your professional experience than your education.