Try Old Navy. They normally have grey skirts, sometimes a-line, but occasionally pleated in the adult women section. If you can sew, you could fix the size, or perhaps the smallest size would fit her?

I don't know how much accuracy you/your daughter care to put into the costume, but the skirt for the more recent movies isn't pleated like the one for the first movies. The first two movie's skirt was the "normal" pleating all the way around, in a dark charcoal color, darker than the sweater. The more recent version is lighter than the sweater, and has bigger pleats that aren't all the way around. I have pictures of both, if you need them. If the perfection isn't important to her, any grey or black pleated skirt should work fine. The sweater is really the most important part, imo, few non-Potter fans are going to realize the skirt isn't perfect, but most have seen enough of the movies to be able to tell the sweater is off.

Websites that specialize in school uniforms will be most helpful for the skirts. French Toast and Buckhead School Uniforms both have some good versions, and should be easy to ship to you.

As for the ties, try Alivans or WhimsicAlley. I have an "official" one that came from Alivans that is an adult size, and is huge for me (I'm 5'3), so I bought a kids size clip on one that was navy and scarlet, and just painted the navy gold with fabric paint, it looks pretty nice considering it only cost a few dollars to do, and is mostly just under the sweater, so I don't think perfect detail is important with that.

Let me know if you need help, or have any questions, I had a lot of fun perfecting my own costume a few years ago, and it's nearly perfect for the first 2 movies' Hermione costume, and I'd be more than happy to help you find anything you still need forums ! I got both my skirt and grey sweater at used clothing stores, and fixed both up, it was easy, and my costume was relatively cheap (I don't think I paid more than $30 for everything, including used Doc Marten black mary janes, aside from the cloak), and it was fun to customize it myself. I need to make one for the more recent movies soon, so I've been looking at websites trying to find similar items.