I've always been confused regarding the cache and high price when one purchases Creme Fraiche.
I don't use it very often, but when I do, it is made in my kitchen within 30 seconds. I've read recipes that specify "Never to use cream that is ultra pasturized, or the Creme Fraiche will never form". I've also read recipes that will tell you to use a high ratio of buttermilk to cream. I've found both of those statements to be unfounded.
I take one cup of heavy cream---any brand, pasturized, ultra pasturized, and pour it into a jar. I stir in 1 tablespoon of buttermilk, and then loosely cover the jar. It sits in a warm place for 24 hours (the cabinet on top of my fridge works great). After a day, I give it a little stir, pop it into the fridge, and a tasty, tangy , true Creme Fraiche is the result. When I put it into the fridge, I let the cover stay off for a little bit, to avoid any water condensation. Then I put on the cover. I'm told that Creme Fraiche will stay can stay in the fridge for 10 days.
Have a good weekend