How many patterns call for it?
How popular are those flies?
I'm sure it relates to demand, the local shop stocks what they can sell locally. I used some JC back in the '70s (very few) on the east coast and haven't since then. I could not see that they would have improved the bass bugs, woolly buggers or soft hackle flies much; so the local guy would have lost money by stocking them for me to buy. By the same token a farmer would have difficulty selling a great many skins so not a lot are produced.
A quick search shows a house in NJ and one in Pa that carry them, if you don't like the ebay thing. I'll bet your local guy can order some in if you are in need. I asked the guy near me once and said it take a week, and named a price; I passed, because it was just curiosity.