My son (3) has SPD which was diagnosed at 18 months. He currently has speech therapy, occupational therapy, physical therapy (core strength) and we will be starting mom and child therapy to help with separation issues. I live in Chicago so my references won't help directly but they can guide you.
If your child is under 3 then speak to your pediatrician. He will refer you to EI (Early Intervention) for a diagnostic. They will then recommend which therapies he needs. Once I got my EI diagnostic I also went to the University of Chicago for a second opinion which was the same. Your insurance will cover all of this. I had another diagnostic at age 2 (Easter Seals), one at 3 (Illinois Masonic) and one at 3 1/2 (Erickson Institute) because the one I got at 3 was not thorough.
First I need to warn you about therapists given through EI. You have to remember that this is a federal agency and as such the people aren't always the pick of the litter. Once you do get a good one, they will likely leave after a brief period of time for a better job. So have your wits about you and do a lot of reading. You will quickly be able to tell the good therapists from the bad ones. Be an advocate for your child. You have no idea how many EI therapists I fired. Just keep going until you find the right fit.
As far as places to go for a diagnostic. All decent places will give a team approach but how they go about it will differ as will the services, referrals and recourses they provide after which can be zero or many. For me the University of Chicago (highly acclaimed) and Illinois Masonice (both hospital systems) where the worst. You walked into a room having never spoken to anyone beforehand and are greater by a team of 5-7 people (OT,PT,SLP, Developmental Pediatrician, psychiatrist, social worker, etc.) You sit down and one group peppers your with questions while the other group puts your child through the paces. Then they switch groups. Its an hour to 2 hours long and you are trying to answer every question accurately and quickly (because you are obviously being rushed, because they do these every hour or so). At the same time your little one is over there being asked to do this and that and he's looking to you for help and encouragement. Then you wait about 20 minutes and the team gives you an assessment right there and then with suggestions about next steps. Its stressful. I don't know how they can be very accurate in such a short and hurried amount of time.
At Easter Seals it was a 3 appointment approach. First they sent me a form to fill out. The first day was an hour with the social worker, then an hour with the SLP. The second day (the following week) was the OT and PT. The third day was the wrap up session for an hour. This was much better and more thorough.
At Erickson Institute it was a 5 appointment process spread out through 2 weeks.First they sent me an extensive form to fill out and asked for all previous diagnostics and therapists reports. The first appointment was just me and the psychiatrist. She had read every single thing I sent in and we talked for an hour. She said it seemed like there was still more to go over so we met the next week for another hour (wow). The second appt the psychologist came to my sons preschool to observe him in that environment and had his teacher fill out a survey of her observations of him (wow). The third appt was back at the offices for two hours with the psychologist and social worker to for social interactions and play. The fourth appt was an hour with the OT. The fifth appt was an hour with the developmental pediatrician for behavior and health and an hour with the psychologist again for intelligence testing. The final appt was with the developmental pediatrician (who gives the diagnosis) and the psychologist for the wrap up and next steps. We just finished this past Wednesday and boy was it worth it. (This all went through insurance just the same as the others.) They had recommendations, referrals and resources. They are there to help me and Ive already called twice with questions. This is what they do.
So Ive been around the block with this as well as the public school system (horrible) and private therapies (also covered but so much better). Ive also vetted out the pre-schools and therapeutic day schools (in case you have questions about any of that).
There is an Easter Seals there in San Diego as well as Rady's Childrens Hospital - I would start there. Im not sure if the California school of professional psychology does Autism diagnostics (which is what you have to ask for since SPD is a spectrum disorder) but give them a call and ask. The university of california would likely have a dept for this but in my experience the university health systems aren't great. Always ask what the process is and find out if its a wham bam thank you ma'm or a thorough process. There is typically a very long wait so call everyone and ask each one where else you can go if the wait is to long. Thats how I found Erickson.
Good luck! Let me know if you have any questions!