A ball and socket armature? You might be better off buying the full armature than making your own ball and socket - most of the parts will have to be made yourself.
Of course, since it sounds like you're just starting out in stop motion, it might be better for you to make a cheaper/simpler armature. Start out by making a wire armature like this one:http://www.youtube.com/watch?v=KkEcCZQeTyM
Good luck!

I always use cutted chopsticks with metal wire wrapped around to connect each part of the body and then I apply metal wire in the joint areas.  A ball and socket armature sounds better though, and you may want to go with Jesse's advice.

I saw that tutorial after I posted that comment. That technique for an armature looks a lot better than the wire armatures I've made in the past. With rusty old wire cutters and a nasty old drill, it massacred my hands to make them. And ever since then, I have been afraid to even work with armatures. This will probably save me money instead of having  having to buy expensive tools. So... Yay!