One thing to be aware of is the DM stays on the boat on SoCal dive boats. And they don't do a lot of hand-holding, they figure you signed up because you were able to do the dives. I'd bring/buy some sort of DMSB/Safety Sausage also because if you get blown off the rig they're not going to un-hook and go after you until everyone else is back on board.
It is really cool diving though, big pelagics feeding right in front of you...
Scuba.com is one of the larger online retailers in the U.S. Their showroom/warehouse is in Irvine, Ca (Orange County) If you're staying in the L.A. area and planning to either drop down to dive around Laguna (Shaws Cove etc.) or San Diego, you could detour past it pretty easily. I don't actually know what's inside the store as I've only drove past it. You've got to assume if they have it on their website that someone would be able to walk in back if they have what you want.
Scuba.com Retail Store in Orange County, Irvine
There's also a lot of Sport Chalet's in the LA/SD Metro area. Different levels of dive gear inventory though. We have one here that closed their scuba dept. completely, another that's about 1/2 the store.
Scuba and Snorkel Gear - Sport Chalet
If you do get a chance to head down to San Diego for the Kelp, Marissa, Humboldt or Lois Ann dive the Point Loma Kelp Beds pretty often.
California Dive Boats : The Official Page It's pretty cool diving, you never know what you'll come across traveling through. We've seen all sorts of sharks, tiny lethal cone shells on the stalks, the usual fish etc. Once my buddy encountered a whale out there. Don't surface in the Kelp and think you're going to swim back to the boat - that's a huge mistake. Drop down and stay near the bottom - it's not very deep - when swimming around if possible. If you get caught up in it, don't thrash around just reach around and cut/break it off. You'll notice the boats moor off the edge in clear water - there's a reason for that.
When they're not doing that it's often Wreck Alley. Yukon or Ruby E. are good.
They do go to the Coronados Islands occasionally also but the Kelp there is all gone, overfished on the Mexican side so once the fish were gone, the critters that cut the Kelp holdfasts took over and decimated the forest. Shame - it was lush 30 years ago - I certified there - now it's barren. The Sea Lion rookery is still there though - that can be a whole lot of fun. They'll go after anything bright/shiny, pull on things, get face to face etc. There's some videos on Youtube.
All about San Diego/La Jolla diving -
Divebums - A San Diego Dive Website