I got mine mail order from Vermont Soapworks -http://www.vermontsoap.com/
I'm sure you can buy it locally at crafting supply stores too. It's a fairly common soap base.
I'm loving my homemade shampoo. My skin feels wonderful, it's not tingly / sensitive like it usually is. You just have to rinse afterwards or it's a bit sticky.
My rosemary and lavender seeds are starting to grow so soon all I'll need is the soap and water, so it will be inexpensive!