The Fremennik shield is a shield commonly worn by Fremennik warriors. To wield one, the player must have completed The Fremennik Trials quest. They can be obtained as a monster drop from Fremennik market guards, dagannoths (only in the Waterbirth Island Dungeon) or Fremennik warriors. A Defence level of 30 is required to wield the shield.
The stats of the shield are identical to those of an adamant kiteshield.
Bardur, located in the Waterbirth Island dungeon, will trade you one shark for a Fremennik blade, a Fremennik shield or a Fremennik helm. However, it is usually more cost-effective to high-alch them, due to how out of the way Bardur is, and the value of sharks. This is still useful to players who are slaying dagannoths, as trading it for food can extend their trips.
Bardur will also sell players a shield for 150,000 during The Fremennik Exiles. He can be charmed into giving the player one for free, if they are wearing the ring of charos(a).
For an exhaustive list of all known sources for this item, see here