As far as I know, private investigators are pretty much your only way to go. After my dad up and left about 10 years ago to the Philippines, and with a couple bills to our name after we believed he came back for a bit, those internet people search things proved useless. They basically gave you a list of publicly available names you could look through the white pages for, although a bit updated as they only come out every once and a while. Waste of money imo. Never went with the PI personally because it costed a bit too much.