Trees are valuable resources and deserve to have professional care. To find a certified arborist, you can visit the following website: www.treesaregood.org. This site allows you to search for certified arborists in your area. The site also contains a lot of information on tree care.
Most of the established local tree service companies have arborists on their staff. Some of the larger nurseries in Orange County have ISA-certified arborists on staff. You can verify that an arborist is certified by visiting the above site.