I'm sure those that have made them will share their ideas. There are plenty here. If you want to buy a mold, http://www.historystones.com/cospmo1.html has them.

There's also a good tutorial at the artistic garden.
http://www.the-artistic-garden.com/garden-sphere.html

You can use beachballs, heavy rubber balls, glass lamp globes from the thrift store, concrete molds sold online, styrofoam craft balls. Billie

I used an inexpensive replacement glass globe for a light fixture that would attach to a ceiling with the globe part pointing down. Of course, you have to break the glass to get the hypertufa out. I sprayed the inside with a vegetable spray before packing in my mix, and put the globe inside a plastic sack for safety when I was hitting it with a hammer to break off the glass. You could also use a cheap fish bowl, but it has a flat bottom so your globe won't be as spherical as the light fixture.