What should I look for in an alternative to crème fraîche?
You can make your own crème fraîche. Just inoculate heavy cream with buttermilk. From The Splended Table:


  Ingredients
  
  
  1 to 2 tablespoons cultured buttermilk
  2 cups heavy cream (pasteurized, not ultra pasteurized or sterilized, and with no additives)
  
  
  Instructions
  
  Combine the buttermilk and cream in a saucepan and heat only to tepid (not more than 85F degrees on an instant reading thermometer). Pour into a clean glass jar. Partially cover and let stand at room temperature (between 65F and 75F degrees) for 8 to 24 hours, or until thickened. Stir and refrigerate at least 24 hours before using. The cream will keep about 2 weeks in the refrigerator.


The results from doing that are so close to "real" crème fraîche that I can't tell the difference unless I'm tasting them side by side. If you want absolute authenticity, you can order the Real Culture, follow their instructions, then use that crème fraîche to culture many further batches of real Crème fraîche.

EDIT: HA! :) Here's Kenji's $.02 on the subject: Serious Eats