Consider the message boards at Rootsweb . . . now part of Ancestry.com.
You can post queries, but be as specific as you can. You may even find living relatives you didn't know you had . . . so be careful. Believe me, it isn't always a good thing.
You can also read others queries, and swap information like we sometimes swap treasure hunting information here. Be just as careful too. Not everyone is who they seem to be.
Met some good leads, even got around the adoption laws in the state of Mississippi. A little knowledge, a little arm twisting, and some "happy accidents" (asking the wrong question to the right person) later, I found my dad's sister alive and well. She had been missing from the family for 65 years.
He had found all the other siblings when he was a young man, but was certain she had died. As a result of one lead on Rootsweb, he got to speak to her in the last months before he passed in 2001. If they hadn't botched his heart surgery, he would have gotten to see her in person.