There are multiple references, but I know one is the April 15th, 1865 edition of the New York Herald, in which, a reporter identifies Lee's orderly as Joshua O. Johns of the 39th Battalion Virginia Cavalry. There are some sources that identify George W. Tucker as the orderly who along with Colonel Marshall accompanied General Lee to the McLean house.

Joshua Johns was also a member of "Stonewall" Jackson's staff and was wounded at the same time as the general was mortally wounded at Chancellorsville. He also was an orderly to Richard S. Ewell.

The links reference Joshua O. Johns as General Lee's orderly at Appomattox.

http://www.nps.gov/apco/the-meeting.htm

http://books.google.com/books?id=5JjfNYEJG98C&pg=RA1-PA274&lpg=RA1-PA274&dq=%22joshua+o.+johns%22+appomattox&source=web&ots=QpjP0hWwMf&sig=76zO_IykSF1Pyz7KV4PS9xrVSTk&hl=en&sa=X&oi=book_result&resnum=3&ct=result

Jim Martin