Katsuobushi or katsubushi is a rock hard dried bonito flesh. When I was a child, my grandma often asked me to shave it from the block, before preparing food using a wood plane like tool. Now Japanese people usually buy already shaven katsuobushi.
The flavor of shaven katsuobushi deteriorates quickly once it’s exposed to the air. The best way to get fresh katsuobushi is to shave it every time before using. But the shaving tool is hard to get, and solid dried katsuobushi block is even harder to get. (I am surprised to found out that you can now buy them at Amazon.)
If you buy a big package, divide them into several small ziplock bags, seal tightly, and keep them in a freezer. The less you expose it to the air, the better. Shaved katsuobushi won’t be frozen.
Katsuobushi can be found at Japanese grocery stores and other Asian grocery stores, but if you want to it order online, here are some of the links;