I have never heard of a Graham , but a hydraulic pump coupler should be something you could get at a hydraulic supply shop , could you discribe your coupler or post a picture of it and the machine too and possiblly I could tell you if it is available from an after market source and not jut a OEM part -----I'd picture it could be a ring of plastic type material with a splined hole in the middle of it that mates with a splined hub on the pump like in an excavtor , or maybe two three nubbed hubs with a rubber absorber in the middle or even a solid collar that is splined-------------next question would be why did it fail as you might have bigger problems