A local Shell distributor, if they stocked the right oils, is the best bet, but for what it is worth I bought my lubricants from, of all places, Maine Potato Growers, Inc. by phone. They list both the Dentax and the Spirax on their website. They were more than happy to ship the containers to me. I have not checked with them to see if both oils are still available.
Maine - Texaco Lubricants, Gasoline, Fuel Oil
The Spirax I bought (by the quart) was the EP variety, and the Dentax came only in 5 gallon pails, as Jay has pointed out.
George