Instead of buying a rosemary seedling from your local nursery, you can grow some yourself fairly easily by taking cuttings from an existing plant.
Choose branches that are green and lush, with healthy-looking young leaves pushing out at the end. Cut off about a 5 inch long sprig (the stem should be springy and greenish, not woody). Peel off the leaves at the lower half of the cutting.
Have a pot with moist potting soil ready. Poke a hole in the potting soil with a pencil. Place the cutting in the hole up to the leaves and then press down on the soil so that it is firmly in contact with the stem. Water the cutting thoroughly and place the potted cutting in the shade or indirect sunlight. Keep the soil watered and moist for the next few weeks. You will notice some new growth on the cutting in about a month (or less) as it starts to root.
You can move your new rosemary plant into direct sunlight at this point.
Many other herbs can be propagated in much the same way. These include thyme, oregano, mint, rue, basil, sage… and many others.