Head to Route 39, and head into the Miltank Farm House.
You will find two girls surrounding a sick Miltank. :(
In order to make this Miltank better, you need to feed it five Oran Berries. Feed the Oran Berries one by one, and once you have fed five the girls will give you the Seal Case.
I think they will also give you some seals, I'm not sure though. But there you have it. :D
Hope I helped.