This is not idle fantasy. Once in Berkeley, CA there was a store called Straw Into Gold that shut down in the late 90's and is now just a wholeseller.
Anyways, their retail store had a huge selection of yarns that were all on spindles. Using their handy ball winder, I could spin off as much as I felt I needed, and then buy the yarn by the ounce. It was great for experimentation with color-work. Anyways, I would love to once again find a store that sold yarn this way.