The St. Louis Genealogical Society has some wonderful information on citing sources when documenting your genealogical findings.
Citations: St. Louis
[Name of Cemetery]Cemetery Office (St. Louis, Missouri) lot owner cards, lot [number], section [number], owned by [name of owner]; showing the grave site of [name of person buried] in grave [number]; photocopy courtesy of the [Owner of Cemetery], [Name of Cemetery] Cemetery.