at hardware stores and places like Home Depot or Lowe's, but go to a plumber's supply store, and they'll hook you up. Being in the heating & cooling business I know what's available at my supply houses, and I then I walk the aisles of Lowe's or Home Depot, and I don't know how the average home owner gets anything done.
I know years ago most supply houses didn't like to deal with the general public, but now a days if your money is green they'll usually help you. Just go in, and tell one of the counter guys what you're looking for, and he'll probably give you a hand.