Visitation unco-operation

My fiance and I have been living together for 3 years, he previously had a relationship, and the outcome was a beautiful daughter. He has not seen his daughter for 3 and a half years, and this is really tearing him apart inside. We have an infant son together, and I have 3 children from a previous marriage, so we have a pretty big loving family, that we would really like his daughter to be a part of. His ex lives in another state, and my fiance pays child support to that state, but nobody there seems willing enough to help us with the whereabouts of his daughter, and we both have made repeated efforts, to gain any kind of knowledge, of his daughters whereabouts, and welfare. We are getting really frustrated, and don't know what we can do...we really don't have a-lot of excess money, as we support 5 children, but we desperately need to know if his daughter is in a healthy environment, and being cared for properly. His ex was very heavy into drugs, and a very unstable person, she has spent much time in jail, and travels within her home state, to avoid facing consequences. We live two states away, and we really, really need to know, if his daughter is o.k. Can you help us? What can we do? Please, anything, would be helpful. Thank-you so much!