Cheap Joe's Art Stuff - http://www.cheapjoes.com/
An online retailer of art supplies.
Jerry's Artarama - http://www.jerrysartarama.com
Discount art supplies.
Pearl Fine Art Supplies - http://www.pearlpaint.com/index.htm
Great website for supplies and great brick and mortar stores.
Dick Blick - http://www.dickblick.com
One of leading online retailers of supplies for artist.
MisterArt.com - www.misterart.com
Online Art supply store.