They come in many sizes. Honestly, the only reason I know the name is that I have one in front of me for a current project. Found em at a local michaels, though not every michaels carries them. Saw them in 2.75 inches and 3.15 inches. They have other sizes.
As for the mold, another post (http://www.therpf.com/f9/starcraft-ii-agr-14-c-14-rifle-210659/?highlight=cast+no+seams+rifle) member lightz describe how me made a two part mold, filled each half with a bit of resin and slushed it a bit. Then he filled the mold with more resin, sealed it, and slushed further to get the seam sealed. This produced a "seamless" cast that was still hollow but mostly thick in many parts. This would probably be what you are looking for to get a good sphere shape that is still hollow. Might want to shoot him a PM to see how successful it was though.
Acrylic hemispheres are available off the shelf from plastic fabrication companies in Europe and the States.
You just need to do a search for acrylic / plexiglass hemispheres in your locality.
What size do you need?