I don't know just what you want to accomplish (I'm guessing a browning of your barrels) but if you're after ferrous sulfate don't use iron pills.
"Copperas" is an old trade name for ferrous sulfate heptahydrate. This is ferrous sulfate with seven molecules of water chemically bound. It looks like greenish sugar and goes into solution easily, at an acidic pH. The iron in pills is typically a monohydrate, won't dissolve as easily, and has other characteristics which may cause problems.
Most ferrous sulfate is a byproduct of titanium dioxide production, or of steel pickling. Neither is a great source of pure material. The only large scale commercial manufacturer of true, high purity product uses it internally for pigment manufacture. A bit is packaged for sale, in 50 lb bags.
I'd bet the best place to buy small quantities would be a lab supply house such as Fisher Scientific.
