Trying to find my birth-grandparents

I am looking for my birth-grandparents my mother & I have been looking for her biological parents for quite some time. She was born on November 5th 1959 in or around Sioux Falls SD. I have a pretty good idea of what her borthmothers name is but there is some confusion about her last name. We do know that her birthmother did live in the Sioux Falls area for a while after her birth but are not sure what happened to her. My adopted grandmother used to take my mother in to the store where her biological mother worked so she could see my mother i guess it was a sort of open adoption before its time.