1.2 corsa petrol van?

My corsa van will not start at present.
The imobiliser has cut in and I have no fob to switch it of. This is a factory fitted alarm as the vehicle was previously owned by Siemens & I bought this one from auction. The engine is turning over but will not start.
Where do I find this black box / imobiliser solenoid strarter switch?
Does any one know?