This happens to me all the time, I'm all set to start cooking and realize I've run out of eggs or milk. I've remedies this by keeping powdered milk and vegan egg substitute on hand at all times. Here is an awesome article from tipnut.com that lists substitutes for those things we may run out of.
I've listed a few great tips below, but go here for the full article.
Buttermilk Substitute: You can substitute 1 cup of buttermilk with the following quick recipe–In a one cup measuring cup, add 1 TBS of vinegar or lemon juice, then top up the rest of the measuring cup with milk. Set aside for 5 minutes, stir, then use as buttermilk in the recipe.
Baking Powder Substitute: Try 1/4 tsp baking soda & 1/2 tsp cream of tartar to substitute for 1 tsp baking powder.
Chocolate Squares Substitute: 3/4 cup unsweetened cocoa & 1/4 cup melted Crisco (Substitution for 4 ounces unsweetened chocolate squares).