I definitely agree with going for the unique look; you'll stand out and it's another way of expressing your individuality. Do what makes you happy, not what other people are comfortable with.
That said, from an engineering standpoint you could probably pick up a metallic sphere from a supply store like
www.smallparts.com or something similar. I would recommend against any solid material; the density of metal makes it prohibitively heavy.
How much does your eye weigh, and what is the diameter? Based off of that information, you could probably figure out a more stylized replacement. I would recommend a hollow sphere made out of a material that is compatible with your body; chances are you already know people who would be able to tell you what materials would and would not work. Getting the actual product made wouldn't be too hard.
You could also consider getting an interesting pattern etched into the sphere. Something like a crosshatch or a logo would be really interesting.