i dye muslin all the time- one of my favorite things to do- i love creating my own backings- and i do save alot of money when you look at backings going for $18+ a yard...i can create a one of a kind beautiful backing for much much less.
i purchase my supplies from Dharma Trading Company- and since the dyes last and go a long way you do not have to invest alot to create beautiful original fabrics.
2 books i recommend are:
the dyer's dictionary
and
Fabric to dye for.
but you do not have to purchase a book- when shopping at Dharma they have free instruction guides, hints, tips, and all the supplies you need.