Luckily, both buttermilk and baking powder are easily made at home.
To make buttermilk, just add 1 tsp. of white vinegar to 1 c. of milk. Let it sit for about a minute and the vinegar will both thicken and slightly curdle the milk. You'll want to use it quickly because if you try to make this in advance, it can sometimes cause the milk to separate after a bit.
When truly desperate, I've actually used this "buttermilk" in the place of sour cream in baking recipes. Just add in 1/2 c. more flour and you'll be good to go.
Now for the baking powder:
Just combine 1 tsp. baking soda, 2 tsp. cream of tartar and 1 tsp. corn starch; sift together.
Wasn't that ridiculously easy?
Store the baking powder in an airtight container. I picked a few of these cute mini canisters up at Michaels and they've come in really handy for storing spices and whatnot.
Are there any ingredients you've tried making? Did they work or did they flop?