I'm in Alabama these days, but we just moved here this year from Oakland. I agree--it's pretty impossible to find, even in CA. If you're near San Francisco, you can probably find it at the large supermarket in Japantown called Nijiya Market. That's where my wife used to go to get all her Japanese cuisine supplies and she thinks she has seen it in stock there.
However I prefer to grow all the herbs I use anyway. It's a lot cheaper and you'll have an unlimited supply if you are able to keep them alive :-). We bought these Shiso seeds from Amazon and grew them ourselves. Sadly, none of the red shiso sprouted--not a single seed. However the green shiso had an excellent yield, maybe 80%. Shiso plants love a lot of water and direct sunlight, even in the brutal summer here in AL. I imagine in CA you can grow it year round without any trouble.
Hope that helps.