Google says this:

While many people think that buckwheat is a cereal grain, it is actually a fruit seed that is related to rhubarb and sorrel making it a suitable substitute for grains for people who are sensitive to wheat or other grains that contain protein glutens.

In stores, it's most likely close to other kinds of flour at the baking goods. You can also get the full seeds I think, though I've never tried that before.