Best of luck with your searches.
neptunian, one thing that occurs to me is that if Sunny is right, and your biological mother is alive, is that your adoptive parents may have sworn to keep her a secret. This could explain why you believe that they know the truth but won't tell you.
For example:
In my own family, my (now elderly) husband learned later in life that he had a half-brother that he and his full-brothers didn't know about. It turned out that my husband's father, as a young man, had a child out of wedlock. Due to religious differences, he didn't marry the boy's mother, and this son was put into a Catholic orphanage. WW 2 and my husband's dad's legitimate marriage intervened, and the father stopped all contact with the boy's mother. In turn, she made her son promise that he would never try to find his father. My husband grew up never being told about the half-brother's existence. Decades later, the mother died of old age, and the son decided this released him from his vow. Thanks to the Internet and the help of a niece, this half-brother was able to make contact with my husband's family, in Canada, and they welcomed him. Neither my husband's father nor the half-brother's mother were living, so the full, real story will never be known.
But perhaps your adoptive parents made a similar promise not to say anything about your biological mother, if they know who she was. If this is the case, and your parents or other friends/family know how to reach her, just possibly your parents will agree to contact her on your behalf, without releasing any secrets.