Answer
You can buy them at the Cathedral of the Silver Rain in Tallinus for 500 gold each. When used in combat, then stun your enemy for 2 or 3 rounds. More here.
Answer
Currently: Nowhere. Kepbekk Leaves and Kepbekk Elixirs are received only during adventures (and there are only a few such adventures). They cannot be bought from shops, which makes them a very precious limited resource. In the future it might become possible to buy them, but for the moment it's not.
Answer
You can get magical weapons from many places. Tallys sells some. The Crreyn Blade is in Axepath, Hale is bought from Bryiniver, most of the PGs award a Magical weapon or two. See [here] for a complete list of all magical weapons.