Are you sure that it was given to her in Ohio, which was a free state? Sounds like you've checked in the most likely places, especially Boone County, but maybe she was freed in a different county in KY. I see you checked the state historical society -- maybe the Boone Co. Historical Society has it.
A couple of things:
My great-great-great grandfather was freed in Mason Co., KY in 1838. The slave owner and her son filed a deed of emancipation in court and paid a $500 bond. I found out about the deed of emancipation from the Mason County Historical Society.
I researched the minister who married my great-great grandparents. He was freed in the 1820s, also in Mason County and also by a deed of emancipation. He also purchased his father's freedom. He came from Cincinnati and back to Mason County to do this.
I'm giving this info to suggest that maybe you should focus on KY, which was a slave state, instead of Ohio. As others have noted, you may want to do a quick google search to find out if manumission papers could be filed in OH.
Hope this helps.