I have lately come across good recipes, which happen to require tomatillos. Sadly, I don't have a source for tomatillos.   Is there some way to substitute them in recipes? Physalis berries? Underripe tomatoes? If tomatoes, then which kind?   Will it be a result similar to the original recipe, something definitely different, but usable, or so different that it only works by accident?  
Under-ripe, green tomatoes that have a thick wall and medium pulp will work in many substitutions.   It will remain tart however the crispness of actual tomatillos will not be present. There is an almost citrusy effect to the taste of tomatillos that green tomatoes don't quite match. If this is a recipe requiring lime juice and cilantro, I might also add a bit of lemon juice or zest to account for the bitter, citrusy profile. If you are not using a recipe with lime you can probably get away without adding the lemon since I'm guessing the citrusy flavor isn't focal. 