This is a very quick cheat (quicker than going to the shops) for when we run out of sour cream:
100ml double cream
1 tsp lemon juice (I've hear you can use vinegar as well)
A pinch of salt and pepper
1. Whip the double cream until it is stiff, but not like butter. I think of sour cream as I'm whipping to keep reminding myself of the consistency.
2. Add the lemon juice and seasoning.
It's obviously not the real thing, but if the shops are closed, or too far away, then this is a very suitable substitute.