Royal Heaven Farm in Escondido is great. I have been riding with Jess for a few years now. We do AA circuit to county level shows, and have lots of great Hunter and Jumper horses available for lease as well!
eq3019 - the trainers at Showpark all basically run their own businesses out of there so I don't know that there is a complete list anywhere. I can tell you the ones I know off the top of my head.
Emily Esau Williams EE Show Stable
Mark Connoly Concord Equestrian
Liza Applebaum Full Circle Farms
Andrea Hill Fairbanks
There are also a couple of trainers that really just do kids and a couple of dressage trainers. You can PM me if you have any questions about the trainers or the facility as my D kept her horse there for several years before leaving for college last month.