Where can I buy a pre-packaged potting mix (like Al's Gritty Mix, Fafard's 51L or Fafard's 52) in Southern California? (I live in Riverside CA; 60 miles east of Los Angeles).
Why not just make the Gritty? Sure it's extra effort, but it's more satisfying and beneficial for both you and the plants.
Some nurseries have selection beyond what the big box stores carry. Also, Ozbo.com can be a good source and they have a flat rate shipping.
I don't think Farfard's is distributed in California, although I hope someone will chime in and show me I'm wrong. I'd like to try a couple of their products, too.
If I were in SoCal, I'd look into americanhort.com, a wholesaler. They carry Berger's BM-6 and Sunshine Mix 4, both of which sound close. Otherwise, check in at your locally-owned nursery or hydroponics store, and let them know you're looking for something highly porous and/or bark-based, they may be able to point you in the right direction.
I've gotten great results from ProMix soils (either HP out of the bag or BX amended with perlite). They are fine and moisture retentive though, very different from the 5-1-1. But they are easier to source and require no preparation and less watering, leaving me more time to focus on the garden tasks that I find more fulfilling than screening substrate.