We each have our own - bought them second-hand when we first started. Even though we share a car, each driver needs their own dorian.
They only hire them out at the 6 hour relay. There are no hire dorians for "normal" supersprints. I have tried to persuade them to offer hire units, but they don't want to.
You can do one event without one, and they will time you manually. They want you to have one for your second event. They offer a payment plan over (I think) 3 or 4 events.
There is always a market for used ones - someone advertised one on the board at Malalla at a supersprint last year, and there was a stampede to buy it. They are also used for superkats, modern regularity, and other events. There is also a heap around somewhere that aren't being used - MSCA tell me thay have sold 400 or more, and only 100 or so come to events.