They've offered coroplast at some stores, but the way for most people to get it was the online 10 pack shipped to store. When I ordered my 10 pack a few months ago, the store I got it from didn't stock any. Now they stock the single sheets (I wonder if it's because of my order). So it looks like their expanding the availability.
The 10 pack is a little cheaper than the single sheets, but it has gone up a bit to $135 (it was $110-120 when I got mine).
48 in. x 96 in. x .157 in. White Corrugated Plastic Cardboard (10-Pack)-WC4896-10 - The Home Depot