Got Balls?

Okay, I need some advice.

They had a story on the news this morning about sitting on a ball while at your desk being better for your posture, but I was too busy to stay for the whole thing. I've been thinking about getting a ball for awhile now, but I'm not sure if I should go for high quality, high $$ or if any old ball will do. This is important because I am poor. Does anyone know anything about buying balls, and what I should / shouldn't look for? Plus, good place to buy them? Thanks!

PS, We now have CFL lights installed in our apartment. I expect to see energy savings and $$ savings on our next bill, I'll let you know as should as it gets billed! :)