From About.com. Faster than a trip to the store :)
Milk (just under one cup)
1 Tablespoon white vinegar or lemon juice
1. Place a Tablespoon of white vinegar or lemon juice in a liquid measuring cup.
2. Add enough milk to bring the liquid up to the one-cup line.
3. Let stand for five minute. Then, use as much as your recipe calls for.