I couldn't find pomegranate molasses. Can I make it by mixing pomegrantes and molasses?

If you have an actual pomegranate, remove the seeds, cover in water, add a few tbs of sugar and simmer for about an hour. Then strain. It is not pomegranate molasses but it is delicious and I've done this for this recipe. You are basically making a simple syrup with pomegranate flavor.

The pomegranate molasses I have found in Middle Eastern or Indian stores has only pomegranate as its ingredient -- just reduced juice.

I agree with Monita. It's one of those lost in translation bits of nomenclature. But that said, it's not terribly hard to find in gourmet stores or on-line.

Pomegranate molasses is really a concentrated form of pom juice with sugar and lemon juice. Here's a recipe for making your own
http://www.foodnetwork.com/recipes/alton-brown/pomegranate-syrup-or-molasses-recipe/index.html