Section 48B. No institution supported in whole or in part by funds of the commonwealth shall use for its daily needs, milk produced, processed or packaged elsewhere than within the commonwealth; provided, however, that such institution may purchase milk from a dealer who is duly licensed as a milk dealer by a state which permits licensed Massachusetts dealers to sell milk to any institution in said state which is supported in whole or in part by funds of such state and provided, further, that if at any time the supply of milk so produced, processed or packaged is insufficient for the needs of such institution, or does not conform to the required standard for such milk, such institution, while and to the extent only that such insufficiency or lack of conformity exists, may use milk not so produced, processed or packaged. This section shall not apply to cream or to certified milk.