Adding Sour to a recipe

There are two flavors I have a hard time acquiring: sweet and sour. Today, I'm just going to list what I've found for sour, for others in that situation.

Vinegar and citrus are what most folks use for sour. Vinegar is out for me right now, and citrus only in very small amounts, as I have a limited supply from a couple farmers. So, here's a few other things I've come across that may work.