I'm sure I've seen them somewhere! It will either be Dorothy Perkins or Next, although having just checked out both websites I've not found them on there. But they were 2 shops I was in at the weekend, and I definitely saw them in one of them!
To be honest I only saw a quick flash of the girl in question's shoes, so it's hard to say if any of them are exact, but they are all most definitely the style I was looking for.