Solaceon, enter the ruins on the east side of the town, talk with the boy. After that, visit him in the nearby house.
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It is not possible to get the Seal Bag in Pokemon Diamond. The Seal Bag currently has no use and is likely something left over from development. It possibly used to have the same role as the Seal Case but got changed during development.
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You go to a house near the Solaceon ruins with the woman and the kid, and talk to the woman.
Y
You go to solaceon town and go down the ledges to the second house. Then you talk to the people in the house and they give you one.