This is a list of all 26 Secret Shell locations.
You may notice that the sixth and eleventh Secret Shells are both located at (11-I). The reason for this is that the Seashell Mansion is at (11-I). If you go there with five Secret Shells, you will get the sixth. If you go there with ten, you will get the eleventh.
1. (4-K)
2. (2-H)
3. (4-N)
4. (7-N)
5. (6-K)
6. (11-I)
7. (9-K)
8. (5-H)
9. (10-L)
10. (12-I)
11. (11-I)
12. (10-O)
13. (3-N)
14. (5-K)
15. (16-P)
16. (11-N)
17. (15-K)
18. (7-P)
19. (9-P)
20. (7-K)
21. (13-A)
22. (14-B)
23. (13-G)
24. (9-H)
25. (15-A)
26. (2-K)