sub for creme fraiche?

I wouldn't worry about an exact match, the suggestions above should work fine.

McGee has some interesting comments:

http://www.nytimes.com/2009/04/15/dining/15curi.html

You can dilute the sour cream with a bit of heavy cream to tone down the sourness, too. Depending on the recipe, you can potentially use full fat / Greek plain yoghurt.

Better yet, make your own creme fraiche: 1 cup of heavy cream, stir a tablespoon of buttermilk or full fat yoghurt. Sit at room temperature, covered, for 36 hours. Et voila! (Refrigerate after 36 hours)

Not really a true sub, but depending on your recipe whole milk sour cream can work.