La Purisima Mission State Historic Park is rustic (no electricity or chairs for guests), but the gardens and fountain are a lovely and serene spot for a wedding of 100 people or less. Do keep in mind that the park is public, so people are allowed to wander by. You will need to fill out an application form and pay a $250 fee. Their contact information is 805-733-3713.
I visited the park recently and snapped a couple popular spots for wedding ceremonies: