On a number of occasions I have needed to research plastic spheres / globes - either fully round or just half, and upon needed to find yet another, I have realized that I have never included any of that information here.
If your looking for small balls 1/4" to 3" in diameter you can check out tap plastics.
Rosebrand has clear plastic ones, with a large seam, from 4" - 36". They are pretty reasonably priced, if you can deal with the seam.
For a variety of options you can try Plastic Balls. they have a wide range of materials, but tend to be small.
For some custom manufactured globes, try California Quality Plastics. While I am looking for a frosted globe, this is my current best option as I need a 18" globe.
Barnard Ltd. also has large spheres and their pricing is good. This place has a varity of theme decor that is not available at Rosebrand, so it's worth a peek. The artifical drinks and food could be especially useful for props.
Complast.com has a variety of sizes w/ seams and without.
JMK Displays offers small balls and cubes, but also offers acyrlic scrap priced by the pound. This would have been fun while in Grad school, to have ordered a couple pounds and experiment with engraving & lighting, frosting, sanding, gluing and so forth since there are tonsof techniques out there, and they give a variety of effects.
Delvies Plastics has small balls and cubes in qty that are polished.