Here are a selection of links to my favourite manufacturers and suppliers of materials.

Oil paint
Old Holland 
Williamsburg 
Michael Harding 

Mediums
Winsor and Newton
Pip Seymour 
Lefranc & Bourgeois

Brushes
Rosemary and Co 
Escoda
Da Vinci
Pebeo 
Premier
Raphael
Pro Arte

Canvas
Claessens Artists' Canvas
Belle Arti
Russell and Chapple

Easels and palettes
Mabef
Soltek 
Cappelletto
New Wave

Pigments
Kremer Pigments
Natural Pigments

Paper
Fabriano
Canson 
Arches

Charcoal and pencils
Nitram 
Coates Willow Charcoal
Tombow
Conté à Paris
Staedtler

Materials suppliers
Zecchi
Cornelissen
Pegasus Art
Lawrence Art Supplies
Jacksons Art Supplies
Atlantis Art
Green and Stone