This is what everyone asks for and instead of searching through the Atlas for the locations you can just use this handy listing. Just get these in order for the proper effect.

1. In the large bush patch at Mabe Village
2. In a secret room at the Tail Cave
3. In the Doghouse at Mabe Village, dig for it
4. In a chest at the Mysterious Woods, hidden behind a boulder
5. Under a bush in the middle of Kohilint Prarie
6. Go to the Mansion after getting five shells
7. Go to the screen right of the mansion and get the shell under the bush
8. On the plateau above the Key Cavern, under a bush
9. Under a rock at the Hole-Stone "maze"
10. Dig for one by the Owl Statue on the plateau by the Hole-Stone "maze"
11. Return to the Mansion for another Shell
12. In the cave under Richard's Villa in a chest
13. Under a bush in the same screen as the Mermaid Statue
14. Run into the tree left of the Tail Cave
15. In a tree by the Telephone Booth east of Mabe Village
16. Dig next to the Owl Stone at Martha's Bay
17. Under a rock in the Southeast corner of the Yarna Desert
18. Under a pot at the House by the Bay after leading the Ghost to his grave
19. Under a bush in the southern-most part of Martha's Bay on an island
20. Under a bush on the island right of the Key Cavern
21. At the farthest right part of Tal tal Mt. Range under a boulder
22. In a chest at Tal Tal Mt. Range past the room with the five chests
23. Under an Armos Knight at the (South) Face Shrine
24. In a chest at the Rapids Ride, you need to get through the Face Shrine to get to it
25. Use the Rooster to cross the Chasm outside Kanalet castle to get to the Shell
26. In the Eagle's Tower dungeon on a ledge, you need to drop down onto it.