The buckler is found in The Forest of Fallen Giants, just before the ladder to the Cardinal Tower bonfire.
As the description states, the Buckler is designed to be used for mostly parrying enemy attacks, and not so much as a shield to defend one's self with. However, the location it is found in, as well as having the superior ability to parry incoming attacks, makes it an invaluable asset against the Pursuer or other strong, but predictable, enemies.
When fully-upgraded, the Buckler has the same damage (if used as a weapon) and scaling as a fully-upgraded Target Shield, with much less stability.
It upgrades with regular titanite, up to +10.