Section 39. Justices and clerks of district courts, except the central division of the Boston municipal court department, may procure all law books relating to the laws of the commonwealth, including the reports of the supreme judicial court, the Massachusetts digests, all blank books, blanks, stationery and other incidentals required by said courts and approved by the justices. The clerk of a district court in any county other than Suffolk, the judicial district of which has a population exceeding one hundred and twenty thousand as ascertained by the last preceding national census and in which there is no county law library, may, with the approval of the chief justice, procure other law books required by said court. The expenses thereof shall be certified by the justices or clerks monthly, and shall be transmitted to the state treasurer for payment.