Your best bet would be to check out branches of Closet Child, KERA shop and Atelier Pierrot, I would think. I can’t remember seeing any plain black bonnets recently, but they sometimes pop up. Unfortunately, there is nowhere I know of that has something like that permanently in stock.

However, you can order one through Triple Fortune. They do bonnets made-to-order. The only problem is that they take a long time. The usual waiting time is around 1-4 months, but I currently have some items on order for customers that are approaching the 8-9 month waiting period since Triple Fortune are too busy.