How to get appraisal for original charcoal illustration

I need to find out how to get a original charcoal illustration appraised. The artist was William S Nortenheim and he was illustrator for children's books and he was the author and illustrator for an article on President Taft in National Magazine in 1912.

He was from the Bucks County Pennsylvania area. In 1923, he was travelling through my great grandparent's village - Old Zionsville, Pennsylvania. I don't know the complete details - but my grandmother told us that Mr. Nortenheim stayed overnight at my great grandparent's house when my grandmother was 15 years old. As a guest in their home, Mr Nortenheim drew a charcoal illustration of my grandmother and grandparents as a thank you for their hospitality .
It is signed by him, and based on our research it looks to be his drawing.
We also know that his parents are buried at the Lutheran Church, across the road from where my great grandparents home was located in Old Zionsville. We also know that his wife was a known seed catalog artist - her name was Grace Hammond.

If anyone can provide information, place contact us. Thank you.

Susan S