Though you can make do and juggle nearly anything, I really like to juggle with nice, crocheted balls. And when you make your own, you can make them exactly the fullness, dimensions, and weight you're looking for.
This tutorial also works for hackeysacks.
Step 1: Materials
Step 2: Starting
Step 3: The Bottom
Keep expanding it flat until the minimum dimension is 2" and the maximum dimension (point to point) is 2.5".
Step 4: The Sides
Keep going until you've gone up 2".
Step 5: The Top
I marked my 1/6ths with pins as shown.
Stop before you've closed it off too far.
Step 6: Filling
I used quilt batting here. It was the only thing I could find, and I don't really recommend it. It works okay for a juggling ball, though.