Congratulations and best luck in finding a mission venue on short notice.
There is no other option but to get on the phone and call the rectories of all possible missions. San Fernando Rey would be nice but if that proves impossible three other options I would consider are San Gabriel (not as extensive or as elegant grounds but the mission church itself is spectacular and it is close) San Buenaventura (in Ventura) and - a bit further drive for you - Santa Ines mission. Santa Barbara is typically booked 6 months in advance and San Juan Capistrano is also very difficult to book. There are telephone numbers on our website.