Mo. Rev. Stat. § 491.110

Subpoenas shall be directed to the person to be summoned to testify, and may be served by the sheriff, coroner, marshal or any constable in the county in which the witnesses to be summoned reside or may be found, or by any disinterested person who would be a competent witness in the cause, and the sheriff, coroner, marshal or constable of any county may serve any subpoena issued out of any court of record of their county, in term time, in any county adjoining that in which the court is being held.