If you can locate his grave site and have some documentation on his service (either Union or Confederate) the local (probably a county office in Oklahoma) Veteran Service Officer can order a stone, free of charge, from the U.S. Veteran's Administration. I'm not sure what, if any, charges there might be for shipping and for setting the stone at the grave site. I'm sure some of the members of this site could give you some better details on the process.