A lot of arts/craft stores should carry this kind of stuff.
When I bought mine, I got it at Michaels. Places like Ben Franklin or Hobby Lobby might carry it too, depending on what you have in your area.
It's been a while since I bought it, but I think it was in the art section with the oil paints, brushes, canvas, and all that kind of stuff.
The brand I got is called Winsor & Newton.