I did an Internet search for health food stores in southern california. I don't know the area well enough to go further. I'm sure that you can do the same thing with better results because you know the area you want.
Here's a link to the various Whole Foods stores in that area. I shop in Whole Foods markets in Rhode Island and Massachusetts and they have an extensive variety of foods, including grains. You can ask your family to call the one nearest to them to see whether it carries steel cut oats. I'm sure that it does--probably both packaged and in bulk:
Here's the site for Erewhon in LA at 7660 Beverly Boulevard near The Grove, Farmer's Market, and CBS Studios. Telephone number is 213-937-0777:
Here's the link for the Wild Oats natural markets. Click on the store locator:
There were a lot more, but that should get you what you need. Good luck. :)