Jimmi, I got the Shell product directly from Shell. Most manufacturer's sales reps will be glad to give you a small amount of stuff as a sample if you make it clear that you are trying to find a place to buy it. If the manufacturer has no dealers who carry a product in consumer sizes they will often offer to send some out as a sample.
I have had that experience time and again in my 50 year interest in the car hobby.
The Corn Head product from John Deere does seem popular and I was unaware of it when I called Shell. Today I would simply use that.
