Well, obviously the reason for the Gryff robes is because the focus is on Harry, and he's a Gryff. And they only make so many items for each movie. Have you considered buying a green scarf?
I'm not saying it's ok. I do want there to be more robes, and I've already said I want a Firebolt. But you know. They're busy and they can only do so much.
*cough* Join Dueling Club at the Haven *cough*