Well, there are lots of options when it comes to cork.
You can buy corkbark flats at a lot of local fish stores or terrarium supply stores. The "flats" aren't actually flat, however, since they are taken right off the tree. They look the most natural but they are also more difficult to attach. http://www.blackjungle.com
You can also get corkbark tiles from someplace like http://www.thecorkstore
which are typically 12"x12" panels of natural corkbark that have been flattened into panel form. These look great and are easier to attach, obviously, since they are nice and flat on the back.
I've heard that people using the above two versions of bark will sometimes spend some time with a file or sandpaper to work down the sharp edges of the wood to make it look more weathered.
There's also cork panels, the stuff people use to make notice boards, available at places like Home Depot, Ace Hardware, Lowes, etc.. They come in two colors- the usual "tackboard" brown color and a darker version. While they aren't as natural looking as actual corkbark, they still look really great as a backing, especially the darker version, and they are a lot cheaper than the other stuff.
Cork will float pretty much no matter what you try, so to keep it on the back of your tank, you'll have to silicon it in place. Just make sure to use aquarium safe sealant or your fishies won't be too happy.
Best of luck~