Newstarget came out with an article about soap nuts. They contain soapy substances called saponins. You can use them to wash your clothes and make a general purpose cleanser. They also can be used as a shampoo and skin cleanser; you have to be careful as the nuts are poisonous to eat. But it seems uneconomical to import soap all the way from India. Top Tropicals has seeds and says the tree can be grown inside in large tubs or outside in warmer climates. There are also seeds on ebay.

Apparently there are several different varieties, because I've come up with Sapindus mukorossi, Sapindus detergens and Sapindus saponaria, and there is a native soapberry tree in the Southwest USA (New Mexico, Arizona and Texas). There are actually quite a few plants that have these saponins and can be used as soap. Plants for a Future has a good article about them, and here's and interesting article about several soap plants but mostly the soap lilly.