Sounds as if you are asking where you can get one. I'll assume that you already tried Aquamax and they do not have them.
Mercury oars will work on their HDR series boats.
A little longer (which will give you a better stroke) and the hole is at 19", not 18", but they will work on the Aquamax boats with a pin-style oarlock mechanism.
Shaft diameter is 35mm, like the oars that came tih your boat.