Getting back to the question raised by the OP, I just bought 6 tins of #11 caps at my local Gander Mtn, so they are available in some spots. They were expensive, but that's the way it goes. Any shortages may be localized. They did have #10s at a cheaper price.
BTW, I love flintlocks and own a few, as well as percussions. From my observations at different club shoots, I would generalize that a quality flintlock can be made to fire almost as reliably as a capper, gives almost as quick ignition, and, with proper care, is almost as reliable in wet weather.