The source I use for cotton fibers (2nd cut cotton linter) is www.danielsmith.com The cost is $2.95 lb. + shipping. The cotton comes in large sheets which I cut up and add to the water I use for the clay.
In reading about cotton and the manufacture of cotton seed oil I have found that cotton fibers do not have the same type of coating on the outside as fibers from wood. Wood fibers are used to make paper. Wood fibers have a stiff, water resistant coating which helps the tree draw water from the ground. This stiffens the fibers and makes paper tough and strong. Cotton fibers are part of the seed generation of the cotton plant and do not have this tough outside layer. Because of this difference I can add more cotton to the clay and greater benefit from it. I use cotton clay to make a lattice type of work which is hard to move to the kiln for firing. Cotton makes the clay very hard in the raw state. Moving my work is much easer with the increased amount of cotton fibers that I add. I know this is strange, try it!