I don't know who made them but it wasn't AG but I had a pair of little black button up boats for my Samantha. They were hard to button like Sam's sailor boots were but very cute with her school outfit.
If you like Nellie's dress better you should just get Nellie's dress. I had another doll, not an AG but the same size and I insisted that when I played school, that her and Samantha had to wear uniforms like I had to, and had to have the same school dress. So we ordered another Samantha school dress for her and I had 2 of them. I think Sam and Nellie could swap clothes easily, or wear matching dresses. The only thing I don't like is putting Lissie or a Kit in a Samantha dress or vise versa. They just don't look like themselves.