Summarize:

Find a basic white button down shirt. Look for a gray or black skirt that almost reaches your knees. Pleats are optional. Pick a V-neck vest in gray or black to wear over your white shirt. Select a tie in Gryffindor colors (scarlet and gold) to wear underneath the vest. You could also wear a button down cardigan instead of a vest, especially if you're not going to be wearing a robe. Look for knee-high socks in a solid color, such as gray or black. Wear black or brown shoes in a simple style, such as Mary Janes or loafers. You can find one at academic bookstores, costume stores, or charity and thrift stores. Just be sure to find one in nice condition, as Hermione wouldn't have had ragged or worn robes. Ask any lawyers or academics in your family if they have a robe you could borrow. If so, be careful not to permanently attach a Hogwarts insignia to the robe.
Wear a white shirt. Find a skirt. Choose a vest and tie. Select socks and shoes. Wear a robe.