In my database I have Thomas Dillard Isbell's parents as William B. Isbell and Ann Dillard. One of Thomas' siblings was Zachariah F. Isbell from whom I descend.
Unfortunately, I did not document the source for the parents. However, I did note the source for Zachariah and that was "History of Maries County, Missouri" by King. I copied the Isbell pages years ago but don't have easy access to them presently because we moved recently and they are in storage.