Catalytic convertor


Had a M reg Sportrak for about 3 months to say i love it.I began to hear a slight rattle about a month after i bought it,but thought it was the undershield.
The rattle has got pretty noticable now,and a couple of garages have told me that the cat has gone.
Will driving round with this broken cat damage the engine?
Does anybody have a decent second hand one that i can purchase....they cost an absolute fortune new