Soba noodles (buckwheat noodles) are common grocery items in Japan, but buckwheat flour itself is harder to get, due to much lower demand.
In the United States soba noodles are hard to find, but buckwheat flour itself is pretty common. It is used for waffles, pancakes or other baking purposes. I found a package of buckwheat flour easily in the organic section of my neighborhood grocery store.
Japanese buckwheat is usually husks and bran removed. Depending on how much the kernels are polished before being milled, the color and flavor is different. The more polished, the whiter the color, and the flavor of the noodles is more delicate.
But the buckwheat flour sold in the United States seem to be made from whole grain. It has more nutritious value and stronger flavor.
Buckwheat flour is perishable. Once you open the package, keep it in sealed air tight container in a cool dark place. Finish using it in 2~3 months after opening the package.
Buckwheat Flour can be found at regular grocery store’s organic section, or organic stores. If you want to order online, here are the links.