There is a Yahoo Group devoted to using natural dyeing:
http://groups.yahoo.com/group/NaturalDyes/There are also lots of books on natural dyeing. You could google "natural dyes". I have many entries in my bookmarks - I must have got them through google and/or the yahoo group.
I took a class on natural dyeing and printing from Michele Wipplinger (Earthues in Seattle) at the Coupeville Arts Center a few years ago. Fabric has to be mordanted in various substances (depending on the type of cloth) so the natural dyes will work. It's not as straight forward as dyeing with procion mx dyes. Cotton, for example, has to be cooked a couple of times with the mordant, and most natural dyes must be "cooked" with the cloth. In class, we cooked in the microwave.
My favorite in that class was painting the cloth with soymilk and then applying various colors of clays, mixed with soymilk; the fabric then has to "batch for up to three months for the clay to be washfast.
Earthues (
http://www.earthues.com/) has natural dye extracts and clays.
I saw a wonderful earthy tiedye t-shirt on the show "Lost" last fall; I wish I had recorded that episode. It looked like it was done in muds, but It was probably mx.
k. taltre