I'd probably try contacting a hire company. Whenever I visit them they have trays of old stuff they never get around to fixing. Specifically I've seen some at Origin Cine in Bristol not so long ago, but in London try Shoot Blue, One Stop, Genesis, maybe Grip Van.
I'm looking for a Manfrotto magic arm, the 143 model with the lever rather than the one with the knob.
It's for use in my workshop not photography so really I'm looking for a pretty knackered old one, the cheaper the better.