After the selection and acquisition of the sites, the county commissioners of the counties may erect and construct suitable and necessary buildings thereon, repair any buildings already erected and equip the buildings for use and occupancy. All buildings constructed in pursuance of this subchapter shall be plain and inexpensive in character. The labor in constructing such buildings and improvements and facilities shall be supplied by the persons committed to county correctional institutions in the counties or transferred thereto from any county correctional institution, so far as found practicable.