Finding good Harry Potter robes is about as easy as getting past the Whomping Willow, possible, but very difficult and the results not always perferrable. First of all, the only robes you can get have the Gryffindore crest on them, assuming everyone wants to dress up as eiether Harry, Hermione or Ron. For those of us who associate with other houses, we're out of luck. I'm trying to locate a set of pattern for making your own robes, but haven't succeeded yet. Below are a few websites where you can get Harry potter robes, hats, scarves, etc.

The WB Shop of official merchandise has some cool stuff, especially when it comes to hats. They have the student hat and the sorting hat, check it out.

Hollywood Costumes has the best robes I've found. As stated above, they only come in Gryffindore, but there are other cool robes here, like Snape's trademark black.

Garb the World is the best place I've seen to get those two colored scarves everyone loves. They are the only site to have scarves for all four houses in thier proper colors.