All promotions in the fire or police department of any such municipality shall be by the chief of each respective department, by and with the approval of the commission, except for the office of chief of these departments, which offices shall be filled by the commission, subject to confirmation and approval by the city council. But no person shall be eligible for such appointment as chief unless he has been a member of such department for a period of at least two years prior to such appointment, except that if the commission is unable to secure from the personnel of the department an acceptable person, then it may apply to the city council for permission to elect such officer or officers from outside the department, and, if such approval is granted, the commission may name such official from qualified applicants outside the personnel of the department.