Cyrus A. Peterson was extremely adept at convincing Missouri Union participants in the Civil War--particularly southeast Missouri participants--to provide a written record of their experiences. Once he got Wilkinson going, he kept him going.... And going.... In my book on Hildebrand I provide a background on what motivated Peterson to pursue the gathering of these memoirs. Ultimately, he became the head of the Missouri Historical Society in the early 20th century, which helped him in his work. Your go-to source for Wilkinson's original writings is definitely the Missouri Historical Society. While they are indexed, the Peterson Papers have never been thoroughly explored by contemporary researchers, so there is no telling what you might find. If you are especially motivated, it wouldn't hurt to spend a day or two there going through and pulling out Wilkinson's writings, or anything else that might be related to him.