I first heard about Soap Nuts when I was watching a different product, the Yirego washer. An electricity free washing machine to avoid hand washing?? Apparently Soap Nuts work in it too. Anyone else heard of it?
In any case, last time I looked into soap nuts I'm pretty sure they can't be grown in North America because the species is really invasive? It's best to buy the dried husks separately. In bulk, they can actually be really cheap. But all of the comments below show that I've been misunderstood. I've been using Soap Nuts not only for laundry, but if you boil the husks into a liquid detergent it's a great multi-purpose cleaner, too. I don't like the smell of vinegar, so this works best for me.
This is that washing machine I was talking about: