Having searched for the best places to get a robe for the amount I was willing to pay, I know what you're going through. If you go to this
website, it will give you a list of places you can get the robes from, or how to put them together yourself.
Just click on the links to choose either the PSSS/CoS or RoA/GoF style robes and go from there.
(The link or Wizarding School Robes is incorrect. Click here