To find Mantids, look for places with lots of smaller bugs. These are ambush predators - they stand still and wait for their prey to walk within striking distance of their amazing spiked front limbs. Good choices are lights at night, flowering and fruiting bushes that are attracting small insects, and anywhere they blend in. While tall grasses often harbor them, such places also have ticks - best to stay in the open and watch carefully.
Other Stagmomantis species I believe are in California include these: