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Heart Gold and Soul Silver: Bill's Grandfather
In Bill's original house in Cerulean Cape (Route 25) you can find Bill's grandfather. If you bring him the Pokemon he asks to see he will give you its evolutionary stones. Best of all, you get to keep the Pokemon as well as the evolutionary stone.

Pokemon Shown
Item Received

Oddish
Leaf Stone

Growlithe
Fire Stone

Jigglypuff
Moon Stone

Pichu
Thunder Stone

Marill
Water Stone

Bill's Grandpa is an easy way to get some evolutionary stones. Unfortunately, you can only get these stones once from Bill's Grandad.

You can find guides to other side-quests in the Heart Gold and Soul Silver section.