Your uni Grad office should tell you the gown company. Ask them about times.
Your contract is probably direct to the company. The Uni will be getting a percentage back from the company
Be careful about the phrasing of the question to the company and keep a copy in print. Just ask about the length of the hire period. They don’t need to know that you are splitting the hire (assuming the gowns are the same) Many graduations have 1000 students and need the gowns back after lunch for another 1000. Otherwise they require 2000 gowns. Obviously there are spares but the logistics are worked out carefully.
You might think that a short hire period is stingy, but at the end of the day, if they need twice as many gowns, somebody is paying