I get it at Target.... if you can't find it, Judy-TX posted this recipe to make your own...
Place 1 cup whipping cream in a small saucepan on low. Heat until it is just barely warmed. Do not overheat or the crema won't thicken. Place warm whipping cream and 2 tablespoons buttermilk in a glass container. Cover and let stand at room temperature (about 70°F) for 8 to 24 hours, or until very thick. Stir well, cover and refrigerate for up to 10 days.
If your kitchen is warm, you can skip the step of warming the whipping cream. If you don't have buttermilk, you can use two tablespoons yogurt with active cultures.