Buttermilk substitute for making creme fraiche?

I want to make creme fraiche. I have 2 cups of heavy cream, but I can't find any buttermilk in my Country. I've read that it can be substituted with lemon juice, or vinegar mixed with milk. The problem is that for creme fraiche you need the bacteria from the buttermilk right?

So how can I substitute my buttermilk for this recipe?