You find someone who will give you what you want. You might check your state's organization for landscape design and installation professionals. In Colorado, for example, it's the ALCC, Associated Landscape Contractors of Colorado. The organization has a list of members that may be posted online. That would be a starting point for you. You could also check ASLA, the Assocation of Landscape Architects. A landscape architect will likely be more expensive than a landscape designer. But if your project is relatively simple with no major drainage issues or tall retaining walls, a landscape design and build company could probably handle your project.

I suggest you contact the San Diego Horticultural Society and attend a few of their meetings. Network a bit with the members and you'll probably get some decent recommendations.

there is tons of talent in your area.
I also second catkims recommendation to check out the local hort society.

off hand, I've been enjoying the work that local San Diego designer Debra Carl has been posting to the Houzz.com website.

she's quite talented : http://www.deboracarl.com/

Here is a link that might be useful: deb carl's website