There are a lot of alternatives to make your own shampoo at home and many of them use castile soap (because it doesn't involve any kind of animal fat in the production process). You have a few examples here and here.
The idea behind all of them is pretty similar, mixing castile soap (I am pretty sure that other kinds of soap can be used but this is the one I have tried and I'm familiar with) with distilled water to get a nice texture and with herbs and essential oils for aroma and other properties (chamomile extract is used in many industrial shampoos).
If you don't want to mess around and make your own shampoo there are also a lot of companies that make natural grooming products of all kinds (you can even often find some natural cosmetic projects in Kickstarter 1, 2). This products are made with natural materials without lots of chemicals, so I would also recommend you look around for "natural products" and read carefully the ingredients of the products you are interested in to see if they fit or not your needs.