I've seen individual packets with one dose for sale on property, but I haven't noticed anything in larger quantities. They are readily available at any grocery or drug store, if you have a car.
One thing I had never seen until my trip last month, was that they keep a supply of many different medicines at the souvenir shops. A woman came up to the counter and asked if they had any Tylenol, and the CM pulled out this huge tray with dozens of different medicines, all in single dose packets. I thought they only had that type of thing at the First Aid Station. So if you find yourself out in the park and need something, be sure to ask.