You will have a hard time finding southdown in Cali. Most HD out west don't carry it. I would recomend doing searches on every forum you can find.
Pure Caribbean Aragonite is a great product, but more expensive.
As for the silica playsand, you will find 100 posts that make you super nervous about using and 100 posts about how its as good or better than anything else. I have even heard that if you can find the sugar sized pure white silica used for sandblasting that it can be a very good substitute.
My problem is that I am a chicken***t whien it comes to my tank, and I looked to curb risk at any cost. So if you're like me the extra $$ is worth it for the pure caribbean aragonite. But search around for people in your area that might have bought a pallet of the southdown and have left overs to sell. Thats how I got mine.
Dave