I ordered my stability ball from an infomercial (Core Secrets with Gunnar Peterson) and I've been pretty happy with it. Only $10, and I got the ball, air pump, and dvd.
I've seen that they sell balls at Whole Foods, but they're pretty expensive (they're tie-dyed and look cool, but that's not necessary and probably not worth the extra money).
I've never checked personally, but I wonder if sporting goods stores might sell them. There, you might be able to see how one looks inflated and check if it's a good size for you.
Either way, I definitely recommend getting one. Good luck!