Source for cassava or cassava flour
I can't help you with SGV or OC, but I found some at a Brazilian market in Culver City when I wanted to make the Fogo De Chao cheese bread. I think there are a couple of Brazilian shops on Venice Blvd. around Westwood Blvd. One I found online is attached to Cafe Brazil and called "A Taste of Brazil." But I recall going to a different one on the south side of Venice, which seemed to be part of a Brazilian mall, with some other shops. It might be El Camaguey Market. The flour is called tapioca flour, but it is the same thing. They should have sweet and sour.
You also can get the flour from Amazon.com.
though they're both from the cassava root, tapioca flour and cassava flour aren't exactly the same thing. tapioca "flour" is just the starch of the cassava root, which is released when ground cassava is treated with boiling water during processing. cassava flour is made from the whole ground root, not just the starch. you'll see the difference in texture if you put them side by side - tapioca flour/starch is completely powdery, while cassava has a bit more of a flour heft to it.