I have been using something called
J.R. Liggetts Old Fashioned Bar Shampoo.
I threw away the wrapper, but I think that there's tea tree oil in there somewhere.
I have used it off and on for at least the last 10 years or so. I get it at the hippie grocery store (Whole Foods) and have not seen it anywhere else. Might be carried in some health food stores, I'd expect.
Awesome stuff. Cleans your hair without stripping all the natural oils the way most (modern) liquid shampoos do, and you don't need to use a conditioner on top of it. My hair is really soft, and seems stronger and fuller.