Fresh dill weed is widely available in the produce department of most markets. The leaves wilt quickly upon harvesting, but this will not affect flavor.
Spritz whole stems lightly with a fine spray of water, wrap loosely in paper towels, and place in a plastic bag. Store in the vegetable bin of your refrigerator. It should last up to a week and perhaps even longer.
You can also trim the stems, place in a glass with an inch of cold water, loosely wrap the top with a damp paper towel, and invert a plastic bag over the top before storing in the refrigerator.
Fresh dill sprigs can be frozen for up to two months, but be prepared for it to darken a bit in color. No need to thaw it before using. Frozen dill weed will still have more flavor than dried dill.
Dill seed is available dried in the spice section. Store dill seed in a cool, dry, dark place and use within six months for best flavor.