Longing to have another child

I'm a man in my late-forties, creative, artistic, intellectual. I lead a research team at the University. I have a teenager at Berkeley High. I also have a strong longing to have another child. Being a parent has been my most rewarding experience. Although I have had the desire for another child for some years now, circumstances have always gotten in the way, the option to wait always was the easiest. Now I realize I cannot delay much longer. And yet finding a relationship with a person desiring a child is quite difficult in my experience, especially in my Berkeley milieu. The advice I get is always the same -- post something on Craig's list. I have looked over the postings there and every time I get a queasy feeling and go no further.

Time has led me to be very flexible about my goal of having a child. The priority is the child's welfare, whether that leads to a close family relationship, a supporting friendship role, or (least preferred but something I could accept) minimal contact with the mother and child. I could see embarking on this awesome parenting experience  with a single woman or a woman in a relationship who needs help conceiving.

I would be most grateful for advice as to how to proceed.