Modify or replace the arm to the "j-hook" style that is now used throughout the car industry. Once you do that, you can buy replacement blades anywhere. A local marine store (doesn't have to be a Grady dealer since this is not a Grady part) should be able to easily get you the right arm. Take your old arm off and take it with you. I did this on my Sundancer and I actually did it a bunch of different ways, more for experimentation than anything else. The ways I did it varied from using junk yard parts to combining parts of different arms together to buying a new arm that is used on the newer Sea Rays.
But, if your current blade is held on with a small nut, then I believe you can get replacement blades at truck parts stores. I didn't go down this road of experimentation but I read something about a Ford "L" arm (something like that) that is an exact replacement if you have that setup.