If you are looking for very fresh yuba (the Japanese term), you may have better luck in the South Bay. But you could ask at VP Tofu or VK Tofu. They sell fresh rolls made of it (doufu chang), but I'm not sure if they sell it in sheets.
I know you can buy it at the Ranch 99 on Valley Blvd in San Gabriel. There are many different brands. I suspect you can buy it at the both of the places you went, but you didn't recognize it. It most commonly comes in dried sheets.