The design and performance of a cryogenic buffer gas beam source with a load-lock system is presented. The third generation of advanced cold molecule electric dipole moment search (ACME III) uses this source to produce a beam of cold, slow thorium monoxide (ThO) molecules. A feature of the apparatus is the capability of replacing the ablation targets without interrupting the vacuum or cryogenic conditions, thus increasing the average signal in the eEDM search. The beam source produces 1.3 × 10 11 ground-state ThO molecules per pulse on average, with rotational temperature of 4.8 K , molecular beam solid angle of 0.31 sr , and forward velocity of 200 m s − 1 , parameters that are consistent with the performance of a traditional source (without a load lock) requiring time-consuming thermal cycles for target replacement. Long-term yield improvement of ∼ 40 % is achieved when the load-lock system is employed to replace targets every two weeks.