The internet can be a wonderful thing, but also can be very frustrating. As I have come late to making a Harry Potter costume, a lot of links are now broken, or images have gone. So here are few bits that I found useful. I will copy useful portions here on my blog with source links, just in case they disappear in the near future.
One very useful site was by Rebel Haven: http://www.rebelshaven.com/hogwarts.html
She constructed a wonderful Ravenclaw costume, and I am hoping to use the pattern for the robes that she made.
The RPF had some great information too. For the breakdown of costumes for each era:
RPF reference to shoes: http://www.therpf.com/f24/help-finding-harry-potters-school-shoes-something-close-122467