Mr Broadbent's advice is obviously right on. I would add that Chicago Canvas is a great supplier of muslin or canvas, serving the theater industry in addition to photogs. Also, in addition to the rollers for painting, I would suggest trying sponges and brooms to apply the paint with different textures, after some base coloring has been established. Usually after putting a several coats on the backdrop and then doing some test shots, I would want to make refinements such as darkening a corner and or adding some texture. What I found was that, with each successive layer the background seemed to get more interest and depth. So its good to just start rough and assume that adjustments will be made over time. Good luck!