Meadow,
I found my dad after 40 years. It does take alot of work sometimes. If I were you I would start by talking to your husbands oldest closest adoptive relatives. Usually they will have some of the story and may be able to start you in the right direction. They usually know things that have been old family secrets.
Get a date of birth, where she was born, any info what so ever. Your work will be cut out for you. Was she adopted as a baby or older child? It could be all sorts of things. She could have been adopted within the family. The family may have been large and poor, not able to feed another mouth. It could have been incest, rape, affair and swept under the carpet quietly. She may have been secretly born at home and given away with little record of birthfamily. A lot of births were at home back then. See if there were orphanages in the area.
It will help to get an idea of the town and practices in the year of her birth. If you would like to give what info you have, there are alot of people here that like alittle challenge and could maybe help you search. Good luck. I know how good it feels to finish the puzzle.