The best place to buy charcoal is the WKC. ;-)
Get it where you can for now, but keep an eye out on the message boards. When it goes on sale at a major chain store (Home Depot, Lowes, Wal-Mart, etc.) you'll hear about it. Then buy and bunch and enjoy the cheap eats! Stock enough at Memorial Day to make it to the Fourth of July, when you can get enough to make it to Labor Day. But that's when the pressure is on, because you'll need to figure out how much you'll need to make it through the winter!