How do you get a fashion case in Pokemon Diamond?

You can't get a Fashion Case in Pokemon Diamond. That item was introduced in Pokemon Black and White.

However, you can get a Style Case in Diamond, which works similarly. Here's how:

1. Complete the main story: You need to beat the Elite Four and become the Pokemon League Champion.

2. Talk to the old lady in Veilstone City: After becoming champion, visit the old lady in the house to the left of the Pokemon Center in Veilstone City.

3. Receive the Style Case: She will give you the Style Case as a reward for becoming the champion.

This case functions similarly to the Fashion Case in later games, allowing you to customize your character's appearance with various clothing items, hairstyles, and accessories.