There are many online suppliers, some with interesting specialities. Also below are a few general suppliers, and locals who are always worth a look.
Cass art is a well established retail chain based in Islington, London. Their online sales are relatively new but with a huge selection of materials. See the HAs material baskets for best artist materials for particular disciplines.
Cass Art offer a wide range of paints, pens, pencils, canvases etc. nearly all at a good discount price. Their packs of ready prepared canvases, especially the linen ones are good value. They offer fewer high quality choices than Jackson’s but better deals at the cheaper end. Their own brand products are good quality and good value.
The shops are well worth the visit – Islington is a huge treasure trove. Winchester has a CASS store. If you find something on line you can have it delivered to their shop attached to the Winchester School of Art
Jackson’s Art Supplies
This site is supposedly run by artists, for artists. They are very helpful and knowledgable. They supply a variety of high quality paints, and will make up canvases in any size or shape. Their own brand materials are of a high quality and reasonable price. Worth a try.
The physical catalogue is a total joy, worth many hours of browsing.
Art Discount, or Grantham’s Art and Graphic Supplies
This online store a useful source of cheaply priced everyday useful stuff – packs of cartridge paper for example. They are efficient and well priced.
Worth a look to see if they have any discount equipment that you need. Sometimes they have discounted art history books too.
Noble Art Supplies
Art shop in Salisbury that is filling the gap left by the demise of the Complete Artist. Knowledgable owner with many years experience working in art shops. The Facebook site has local news for anyone interested in the arts.