FoFa says:
"I had a recipe calling for Crème fraîche, but could not find it. Yes you can substitute Sour Cream for it, but not quite the same."
Heavy Cream
Cultered Butter Milk or Sour Cream
Crème fraîche can be made at home by adding a small amount of cultured buttermilk or sour cream to normal heavy cream, and allowing to stand for several hours at room temperature until the bacterial cultures act on the cream.