Just in case you can't find creme fraiche at the grocery store, here's how I made it to use in the bran muffin recipe I just posted.
1 quart heavy cream
4 tablespoons buttermilk
Pour a quart of heavy cream into a bowl or other container, stir in 4 tablespoons of buttermilk, cover, and let stand at warm room temperature overnight (at least 10 hours), or until it thickens. Stir it in the morning and it should be thick and creamy. If it is still liquidy, re-cover, return it to a warm spot, and check it every hour or so until it thickens. When it has a good consistency (about the consistency of sour cream), cover it tightly and store in the refrigerator for up to 2 weeks. To make more crème fraiche, you can just add 4 tablespoons of your previous batch to a quart of heavy cream and proceed as directed. The old crème fraiche acts as a starter to get the new batch going, and you don’t need to buy more buttermilk.
Recipe source: Flour by Joanne Chang