Professional art store
Natural Pigments has become my choice for oil paint mediums, modifiers and paint making supplies. They have a large selection of hard-to-find materials, which they have researched and documented for purity and composition.
The resource pages offer need-to-know information explaining when and why to use paint modifiers and the benefits and problems associated with their use. The company’s professional practices classes are designed to educate painters about materials (not something normally taught in art school or college), and it's a wonderful service. They are working to take the mystery out of oil painting and helpfully debunk ill-advised practices offered by many. I have learned more about how to use my painting materials on this website than I did in all my painting classes in college and most of the information is free.