this is a link to one of the same sites which explains that dyeing acrylic can be done but is much more involved and complicated and toxic!
Carla, I have bought some yarn from her and had a good experience so i would def reccommend.
and on this page it explains what kind of fiber reactive dyes you can use to dye cotton
i've dyed some cotton with little pouch of dye found at micheals..i'm not sure what the brand is.. it was a little messier and and i HAD to wear gloves and a mask. the dye took a while to rinse all out, but the color turned out great.
i'm sure there are other dyes out there that would be easier to use with cotton though
hope that helped!