Question
I'm building an extensive raised bed vegetable garden in Palm Desert, CA. Can
you recommend how to locate a good source of quality organic soil?

Answer
Hi Patti,  The type soil you use should be blended to best suit the plants you intend to grow.  "Organic" soil is sort of a misnomer.  In a healthy soil, not everything is organic...by design and necessity.  
For a large area, contact a local bulk soil company.  Talk to your nurseryman about what should be blended for your plants.  Leaf mold, compost, ground pine bark (all organic), sand, maybe some pea gravel (inorganic) and then have it blended to your specifications.
Once delivered, have the soil tested to see if lime, phosphate or potash need to be added.  Jim