Sour Cream

Here are some other substitutes:
Cooked sauces: 1 cup evaporated milk plus 1 tablespoon vinegar or lemon juice. Let stand 5 minutes to thicken.
Dips: 1 cup yogurt (drain through a cheesecloth-lined sieve for 30 minutes in the refrigerator for a thicker texture).

I agree with Pierno, I use it often in place sour cream. There is also a homemade creme fraiche recipe on this site. Somewhere in the blog, I could not find it but maybe someone else knows where it is and can point you to it. I made it and it worked well.

There is probably a Central American/Costa Rican equivalent to Mexican "crema" in markets. It is basically Mexican sour cream.

If you have access to buttermilk, homemade crème fraiche might be a good solution too! http://www.foodinjars.com/2011/02/homemade-creme-fraiche/

It depends what you are using it for. In a recipe you can probably sub plain yogurt. I'd use a full fat or low fat variety. As a sub for a condiment, full fat greek yogurt or even 2% will get you pretty close. And I'm assuming that if you don't have sour cream, you won't have creme fraiche, so I'll sort of skip that suggestion.