Here are some good suppliers of oil paints and medium.
Classic Artist Oils
Triangle Coatings
1930 Fairway Drive,
San Leandro, CA 94577
Phone: 800 895 8000
Web: http://artistoils.com/htmls/default.asp
Winsor & Newton, Gamblin, and Grumbacher oil paints, mediums, and thinners
Jerry’s Artarama
5325 Departure Drive
Raleigh, NC 27616
Phone: 800 827 8478
Web: http://www.jerrysartarama.com
Various manufacturers of oil paints, mediums, and thinners
Lawrence Art Materials
208 Portland Road
Hove, BN3 5QT
United Kingdom
Phone: 0845 644 3232
Web: http://www.lawrence.co.uk/acatalog/Oil_Painting.html