Oracle of Seasons
The Seed Satchel is found inside of the first dungeon of the game, Gnarled Root Dungeon. It can hold up to five different types of Mystical Seeds, including Ember Seeds, Mystery Seeds, Gale Seeds, Pegasus Seeds, and Scent Seeds. At first, the Seed Satchel can only hold up to 20 of each type of seed, but Link can receive an upgrade that allows him to carry up to 50 of each seed.
Oracle of Ages
In Skyward Sword, Seed Satchels are optional items. Small Seed Satchels can be purchased from Rupin's Gear Shop, and hold a maximum of 10 Deku Seeds to be used as additional ammunition for the Slingshot and Scattershot, akin to the Deku Seeds Bullet Bag upgrades in Ocarina of Time. Small Seed Satchels can be upgraded to Medium Seed Satchels (hold 20 Deku Seeds), and by extension Large Seed Satchels (hold 30 Deku Seeds), at Gonzo's Scrap Shop.
Each Seed Satchel takes up one slot in the Adventure Pouch, and can be stowed at Peatrice's Item Check when the extra Deku Seed capacity is not desired. They are useless while Link holds the Cursed Medal.