I'm new at this so I could be wrong, but I believe sour cream is basically cultured buttermilk, but made with cream instead of milk. Directions are the same for sour cream as they are for buttermilk and both use the same culture. So if you have, say, a pint of cream you'd add about a half cup of buttermilk to it and let it culture at room temperature for about 24 hours (give or take a little depending on your house temperature - then you'd have sour cream.
Now, I would imagine, just as you can culture buttermilk with existing buttermilk, you could likely culture sour cream with a little of a current batch of sour cream - makes sense to me anyway.