Based on what I have found and seen found on the Cold Harbor 1864 lines and the Petersburg lines my order of commonality would be tin back block I buttons or eagle buttons. That is not to say that clothing being issued would not have imported buttons or western theater common buttons. It is just that the common block I button is found much more frequently than any other CS button. I have some dug examples on this page. http://www.angelfire.com/ma4/j_mayo/csequip.html

If you can pin down several Petersburg locations other than locations on park property, where the 26th was I may be able to tell you what I found in the area.