Best place for stock sail dimensions is in the class rules, para 5.12 http://www.j30.org/html/technical/2005J30ClassRules.pdf

Fair market price may not be much since the rules have changed on allowable materials permitted in the head sails, and these are very old. Look here for what people are selling used sails on the J/30 marketplace. http://www.j30.org/html/market/boats.html