From Consumer Wiki
California Civil Code Section 1770 requires merchants to stock an advertised product in quantities large enough to meet reasonably expected demand, unless the ad states that stock is limited. The law does not require merchants to give a rain check allowing you to purchase the same merchandise at the same price at a later date. However, many high quality merchants do this as a matter of good business so a consumer may ask for the same item if the merchandise is sold out.
Consumers who have complaints about rain checks which involve fraudulent advertising, may contact their local district attorney or consumer protection agency.