My mother is primarily a water color painter but from time to time she uses other mediums. I forget exactly what she was looking for but found it at Daniel Smith -- we have a physical location reasonably nearby. While the site doesn't offer on line sales they have a link of places that do (which includes a link to Dick Blick mentioned above). (Edit: I noticed when I went back to the link that there is a shop now button I somehow missed before)
http://danielsmith.com/retailers/online-retailers/