Kasoori methi is sun-dried fenugreek* leaves (Trigonella Foenum Graecum). They are used in Indian cooking and taste similar to a combination of celery and fennel with a slightly bitter bite. Typically the leaves are crumbled and sprinkled over meat and vegetable curries before serving.
*Fenugreek seeds are called methi or ventayam.
Look for fresh methi leaves in Indian grocery stores or use dried methi. Dried methi leaves are very pungent. Look for them in small boxes or bags. You can buy the dried form at Amazon.com: Kasoori Methi.
If you don't have kasoori methi you can substitute:
Dried crumbled fenugreek leaves
If you use a lot of fresh methi, you may want to grow your own at home. It can be raised directly in the ground, or it grows well in pots as well. The herb likes warm, fertile, well-drained soil. The sown seed should be kept moist until the leaves develop then it becomes relatively drought tolerant. This is an excellent video that shows how to grow and harvest methi.
If you are using the leaves fresh, the younger they are, the less bitter the leaves will become. Once the plant stalks are 4-5" high, then they will be at their sweetest. Use these young leaves in bread dough, stuffing, or added to a fresh salad. Make sure to use the leaves only, not the stems.
Once the leaves are 6-8," long, they begin to lose their flavor. Typically you would strip the leaves from the stem then cook them with chicken, potatoes, or even spinach.
Your homegrown methi can be dried and stored. The leaves can be sun-dried or dry them in the microwave. Here is a page that describes this method very clearly.