I just bought some buttermilk starter culture, just like I buy the yogurt or cheese culture. You can keep reusing and reusing, but i always start with a culture. Mine will go for months but sometimes I run out, so i jsut use a little bought culture to start it up again. Just put some in raw milk and presto, raw buttermilk. Then use that to sour your cream. If you have creme frache you can use that for sour cream too.