In olivine city you get them everyday from the little girl in the house next to the lighthouse. She will give you different seals each day and some of them are letters. That's what it is in soul silver and I'm assuming its the same for heart gold.
Chat with our AI personalities
go to the lighthouse in olivine city. then, go up and go to the house on the right. talk to the little girl very day and you will get more seals. if you're asking haw to get a seal case, feed ten oran berries to the miltank in moomoo farms, @ route 38.
Go to the Cerulean City Gym!
you go back to olivine
go to blackthorn city and fart
to pewter city