Go to the section near the wagon on the map, then click the bubbles on the water, refuse to dive and brute force the dialogue options until you get it.
nevermind, I see that the wardrobe unlocks regardless of what save you do the achievement on. Question then, does it matter that I don't have the shells for the main playthrough, or are they just something you sell to Dobby?does that mean I'm screwed if I'm already to the part where I can only leave at night with the potion