Hmm it's been 5mths since I had my convocation and yet to take my shot :bsmilie:
Back to topic, there's a shop down @ CQ which my sister says they offers a lot of gowns for rental but not too sure if they will allow you to shoot in their studio.
Also you need to check the insignia's colours and a lot of other stuff before renting the gown, you don't want to end up with a professor's cap and other insignias.
You might want to consider approaching the insitution whereby your subjects are from