If you want small quantities (5 gallons or less) then any chemical supply house (Fisher, Aldrich and others) is your best bet. If you want barrels or totes then you will have to go through a chemical distributor. Big Oil is probably not going to want to do the paperwork on any direct orders of less than a truckload or tank car.
Most official ASTM reference fluids are available preblended through designated suppliers. Check your method requirements as you may have to use official fluids for final performance qualification anyway, so saving some bucks by blending your own for development work may not be worth the hassle.