You can get it at Salacious Sakura, in order to get that you need Carrot charm to create the tales of Sakura. Trying explore all place then you will get the material to create akane costume.
So far I love the game, but I found maybe it kinda bug, after we empty the shampoo, In Bath scene we will be able to see Linda's clearly when she take a bath, but when the story continue, the foam continue to showed up. It won't trigger anymore after the story go.