Erie Cemetery, Erie, Pennsylvania
This metal marker is a replica of the emblem of the National Society of the American Colonists–a shield with an oak tree in full foliage in the center, surrounded by an open wreath of oak leaves and acorns. This society was one of many organizations founded to celebrate American ancestry before the American Revolution.
The organization was founded December 9, 1920 by Sarah Elizabeth Mitchell Guernsey. All members must be able to prove their lineage goes back before the signing of Declaration of Independence on July 4, 1776. Their motto is “Past, Present, Future”.
Gravely Speaking 
Where can I purchase the grave marker for Daughters of the American Colonists?
I would try your local gravestone seller, they may have a source. Or you could try the organization itself. Short of that, I would shop on eBay and see if there is one for sale there.