Kimmie,
In terms of bang for your buck, you can't go wrong w/ a stability ball. Usually, mods are shown to do moves sans ball (haven't previewed push/pull in it's entirety yet so I can't say affirmatively that their is mods shown in that one however. But just add me to the cave-and-get-a-ball,-you-won't-be-sorry camp ;-) ).
Kathy