Edamame are actually not that hard to grow at home. I remember as a kid picking enough to fill huge bowls which we would boil and serve sprinkled with salt.
That said, the frozen ones are pretty good. That's what our kids have been eating. We did grow some last summer in the garden but didn't get seeds this year. Somewhat hard to find.
Restaurants often have suppliers that we don't have access to. There is a local farm, Suzuki Farms, that specializes in Japanese produce that I would guess does supply many of the restaurants with fresh edamame in the summer.