Living in Costa RIca, they used to import Breakstones Sour Cream. Now there is NO sour cream here at all. What can one use as a reasonable substitute ?
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.
If you have access to buttermilk, homemade crème fraiche might be a good solution too! http://www.foodinjars.com/2011/02/homemade-creme-fraiche/
There is probably a Central American/Costa Rican equivalent to Mexican "crema" in markets. It is basically Mexican sour cream.
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.
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).