Hi, I don't know how old you are, but if you're old enough to go looking for your dad, you're old enough to ask your mom some questions, don't leave your mother out of the equation.
Your mother probably has a pretty good idea of how to get a hold of him. She'd know the family and so on. If you're trying to do this secretly though... things could get ugly, especially if your dad hasn't 'grown up'.
I understand the desire to meet and know your family, however, this could cause a world of hurt and your mom could feel betrayed. If you insist on doing this behind her back, be prepared for the consequences.