I had trouble finding a shampoo bar that worked for me too. I happened upon "J.R. Ligget's Old Fashioned Bar Soap," at a local health food store and I really like it. Before that I was filling my own containers of bulk shampoo from my co-op. Hope that helps, good luck :)
I haven't found a shampoo bar yet that I like, and I've tried many. I haven't yet tried the soap Bea recommends because I won't shop at Whole Foods, but maybe I'll see it at Rainbow or somewhere else sometime.
I removed the labels from the unscented shampoo bottle I was using and when that was used up, I had my hairdresser start refilling it with unscented shampoo. Since the pH of (liquid) shampoo is the same as that of your skin, you can use it as soap, thereby saving you from buying a bar of soap. I figured the plastic bottle wasn't going away in the landfill any time soon, so I just kept it. I use my shampoo for my hair and my body. Your hairdresser or any beauty salon has gallons of product sitting around and would probably be happy to refill your own containers. I only use unscented products, which are difficult to find in bulk.
I tried a load of options (including baking soda, bar soap and clay) before landing on rye flour. Sounds odd, I'm sure, but it's the best thing for my fine, straight, hennaed hair by a long shot. I'm no scientist but there is something about it being close to the hair's natural PH that stops all that cuticle opening and closing that loses colour every time.
It's so great that I don't even need conditioner. I just left it air dry and then brush through. With some patience I've dropped from washing every other day to just once a week.
I get bulk fine-ground organic rye flour locally. Put a few tablespoons in a cup and add enough warm water to make it a sort of soupy consistency. Then just dump on your head, massage in and rinse out. Job done. Six months in I have shinier, healthier hair. Not quite as much volume as baking soda but a lot better than the dank, drab results I got from bar soap.
I've tried all sorts of alternatives but nothing has worked quite like plain baking soda mixed with a little water in a cup and dump on my head in the shower. If you do it right, it should feel slick and start to even lather a little. If not, then you're not using enough or your water is too hard.