In the past, you would have had to go to a health or natural foods store to buy these grains, but now you can find many of them at the supermarket. Arrowhead Mills and Bob’s Red Mill are two commonly available brands. Grains—especially whole ones—have oils that eventually turn rancid. Shop at stores where the turnover seems high, and buy only what you plan to use within a few months. If you have space, it’s best to refrigerate grains, but you still can’t keep them forever. You can tell if they’ve lost their freshness by their smell—old grains, including flours, will have a stale odor.