MIMF

I have a 1974 Ibanez Country Classic copy in for repair which needs a new truss rod nut.

The Allen head has been stripped by using the wrong size key.

Does any one know if this is an Imperial or metric nut?

And where I can obtain a new one?

There's some info here: http://www.ibanezrules.com/tech/setup/truss.htm
You'll know the thread size once you get the old nut off. You can try a stewmac Gripper tool or make your own if you have a few minutes. I make mine using a chainsaw file to cut tapered flutes into the next size up allen key (similar to a torx driver but tapered). If you have a set of torx drivers already you can start there. I grind the end off the torx bit to sharpen the corners and then drive it into the hex opening to cut deeper corners until it can get a good grip. Apply some backwards tension to the neck to release some of the truss rod tension and apply a little penetrating oil to the threads via the center of the allen key hole. If the keyhole is a blind hole you can drill a small hole through to the threads to get the oil in.

EZ-out screw extractor