It probably depends on where you really want to work. In Europe you need to do a course that is recognised or approved by the licensing authority and that would be likely to limit you to a European course - I think NATS and DFS offer OJTI courses as well as IANS. I don't know about other regions - I guess it will depend on each individual county's rules because I don't think ICAO has any specific requirement for OJTI ratings or similar.