Most fly shops carry wool now days in good fishy colors.
You can get raw wool from some craft stores and dye your own colors though its hard to get good browns, blacks and olives dying your own or at least for me it is. I have dyed some great yellows, pinks, blues and other colors
Or just use some wool yarn figure eight tied to the hook and brush it out.
I myself use all 3 ways for diffrent colors.