I wish you luck but I've got to ask you a question. Why do you want to find this guy? You need not answer me, but you need to think about this for a moment because you may be setting yourself up for a huge disappointment.
I need him in my life at some point.
No you don't "need" him in your life. I don't doubt that you are curious and would like to have him in your life, but you do not need him.
He's missed all the big milestones in my life.
I hate to tell you this, but it was (and still is) his decision to miss these milestones. He left your mom while she was pregnant with you. Now maybe that was because he was not ready and did not want the responsibility of having a family, but in the years since he could have come looking for you. He did not. And I know this will sound cruel, but he did not want you in his life back then and probably does not want you in his life now.
Look, I get the fact that you would like to know your biological dad. But all he has ever been to you is a sperm donor. (Again, I'm not trying to be cruel, just stating facts.) Do you have a step-father? If so, be happy and content with this man who really has been a father to you. Because the picture you have in your head of your biological father will never live up to the reality if you do find him.
Take care and if you continue to search for him I hope you are not disappointed in the outcome.