Dried pomegranate seeds are used in Indian (primarily Punjabi) and Iranian cooking. The seeds are collected from a wild, sour pomegranate (daru) rather than the sweeter pomegranate, which is typically eaten fresh. The seeds are used to add a sweet/sour flavor to chutneys, vegetable and legume dishes. The whole seeds are included in some lamb dishes. Usually, you crush or grind the seeds in a mortar and pestle before using, but depending on the recipe, the seeds are sometimes used whole. The seeds are also available ground, but you'll get more flavor from grinding your own seeds.
You can purchase this seeds in most any Indian grocery store. They are also available online at many spice shops. Of course you can find them online at Amazon.com: Swad Pomegranate Seeds (Anardana )
If you don't have anardana you can use one of these other souring agents to provide the "sour flavor" (per tablespoon needed)