we recently did the spacesuit for a music video with a bubble helmet, and used an acrylic sphere purchased off ebay. They are sold as light fixture globes, and the neck size and type varies, however if you're making Atlas or anything with an eye, you could likely make it work out. They were less than $100 shipped, and available in black, white, smoked, clear, and also polycarbonate.
If you're going to tool it though, I'd recommend a fiberglass sphere. Not only are they full spheres (no neck hole) they're way less likely to crack or chip, acrylic is honestly a PITA to deal with. I messed up the above mentioned helmet and had to order another one to be more careful with. If you want to see pics of it, check my site in my sig. They are smooth, but the form will not be as perfect as a fiberglass sphere will either.
Here's a link to a good fiberglass sphere supplier, call and ask them for pricing, they were surprisingly cheap! Less than $200 for sure, maybe less than $100 depending on your size. They have a lead time of a few weeks though:
EDON Spires & Spheres
