AI need one for a soccer ball, I figured it might be easier to have only the top half be real cake. But I can't find any half or full sphere dummies, like 6 inches. Should I just use rkt?

it's actually not difficult to make a ball-shaped cake. If you do make a half with a dummy half, you might consider putting the heavier cake on the bottom, so it doesn't tip over. 

anyway, you can get a 1/2 round sphere at a craft store. faster and cheaper.

http://www.save-on-crafts.com/styrofoamball6.html
You could cut it in half, or check out any floral design, or craft store, they will sell Styrofoam spheres. I have bought them at Michael's before.