Where can you get seals in Hert gold and Soul silver other than olovine city?

You can actually get seals in any Pokémon Center in HeartGold and SoulSilver, not just Olovine City!

The Pokémon Center staff will be able to give you seals for free. They are required to evolve certain Pokémon like Poliwhirl to Poliwrath, so it's definitely worth stopping by your local Pokémon Center if you're looking to complete your Pokédex!