Requisition Requests are found in Skyhold, as well as every traversable area in the game. At each of your camps is a requisition officer waiting (sometimes too) eagerly to give you a requisition.
In the field, requisitions typically task you with collecting a certain amount of crafting materials within the area you're currently in. These can be found scattered throughout the world map or in some cases require you to kill certain enemies (like Ferelden locks only drop from bandits or killing Spiders nets you Spider Venom).
Once you have gathered the materials, return to any camp or Skyhold and press X at the "Inspect Requisitions" table. The requisitions you can "fill" appear in white, simply select them and press Triangle to Craft the requisition and complete it.
Note - Requisitions are unlimited. Once you complete one in an area, a new one will be waiting from the Req officer. If you complete a requisition in another area that requisition officer will not give you a new requisition, you must return to the original area you picked up the req for more of them).
Provided you don't mind searching the areas thoroughly you should net this one easily, especially since most of what you are gathering are materials that are valuable in creating and maintaining your equipment and potions.