I had a very similar thing happen to me. I lost about 2/3 of my fish in my 300g. I was not pleased. As stated above NO LFS intentionally keeps their livestock long enough to cure ich if the fish has it when it arrives. If you buy one from SW, even though they medicate their water, a fish would have to be there something like 28 days for it to have gone through the cycle to the point where the stage of the ich would be susceptible to either copper or hypo. (They do copper I believe) I am pretty sure Rob doesn't want to keep his fish that long before selling.
I strongly suggest you take this opportunity to learn that you don't want ich in your tank. Many fish can live with it, as you saw, many can't. Please let your tank run fallow (no fish, inverts are okay) for the next 6-8 weeks. This should rid you of the ich permanently(some people would argue this, but in many cases it works) then QT every fish you get in either copper or Hypo for a month before putting them into the tank.
As I said, this happened to me about 6 months ago, and I let my tank stay fallow since I wanted to keep fish such as Powder blues and blue spots, which I don't think will be able to thrive in a tank that has ich present. You never know what you might want to add in a year or two, and to know that you have no ich because of your preventative measures will greatly increase your ability to add what you want to add without that worry.