Looking for birth sister/sister-in-law...where to start?

Okay DH and I have been in the process of trying to adopt for years now and throughout the process, we've learned a lot about ourselves. We knew prior to our adoption journey that my husband had a half sister out there somewhere, but it wasn't until the past few months that he realized he really would like find and meet her, if she would like to meet us. All we know is she is in her mid 30s (maybe 36), his mom was 16 when she had her and she gave her away to some Catholic adoption agency (I think)? SO...where do I go from here? I have NO clue how to even begin to find her and track her down. She might not wish to know us or talk with us and if not, we'll respect that, but we won't know till we try. Thanks!