Mockingjay.net compiled this list of where you can locate the seals:
District 1: Jewelry Stand in The Marketplace
District 2: Cottage in The Village, House at The Seam
District 3: Gadgets Stand in The Marketplace
District 4: Fish barrel at The Hob, pond in Forest Escape
District 5: Electricity Pole at The Schoolyard
District 6: Terminal in the Capitol Train Station, any Train Car in The Train Yard
District 7: Woodpile by The Lake
District 8: Capitol Couture Stand in The Marketplace, Clothes Stand at The Hob
District 9: Grain sacks at The Bakery
District 10: Prim?s Goat at The Bakery, Chicken at The Seam
District 11: Apple trees in Hideout, Strawberry Plant in Hideout, Tractor in the Village
The District 12 Code: IVOLUNTEER