CA Gov't Code Section 1157.3

Employees, including retired employees, of a public agency in addition to any other purposes authorized in this article, on approval of the governing body of such public agency, may also authorize deductions to be made from their salaries, wages, or retirement allowances for the payment of dues in, or for any other service provided by, any bona fide organization whose membership is comprised, in whole or in part, of employees of such agency and employees of such organization and which has as one of its objectives improvements in the terms or conditions of employment for the advancement of the welfare of such employees, such deductions to be made in accordance with the provisions made by the governing body of the public agency.