Two more methods

You can ask your friends who may know some people including university professors and governmental agnecy employees for letters. Some of the people may like to do this knid of thing. You d better to offer them with drafts if they like to give you hand. By this way, I got three letters from governmental agency research officers, whom I do not know. One more way is you can contact some professors and tell them you ever attended their seminars or met them at conferences, talked to them about your works. You should praise them on their talk with some shining words and say their talk had play important role in your research. By this way, I got two letters from CIT and Harvard among ten professors I asked.