If your recipe requires buttermilk and you realise you do not have any then try making some of your own. All you have to do is add lemon juice or white vinegar to milk and voila instant buttermilk. Some people may add a pinch of salt to this but that is optional.
The acid from the vinegar or lemon juice will react with the milk and make it curdle slightly, this is normal.
Home made Buttermilk Recipe:
1 *Scant Cup of Milk (Whole or Semi Skimmed)
1 Tablespoon of Lemon Juice or White vinegar
* A scant cup is a cup or jug that is just short of being full to the brim. Effectively it is usually 1-2 tablespoons short of being full.