A plateau is generally a result of doing the same types of workouts over and over until your body adapts and stops responding as well. Pretty much any change in your routine will help you to push past it, variety and changing rotations every 4 to 6 weeks is the key to continuing to make progress.
There's no one video, even by Cathe, just is just for breaking plateaus, but finding a video that is different from what you have been doing should be your focus. You say you were doing Firm workouts, which I am not really familiar with, so if you describe which tapes you were doing and what kind of workouts, and for how long and how many times per week, we could help you put together a great new routine.
As for purchasing a step, most sporting goods places and department stores like Target and Wal-Mart all sell different brands of steps. The Reebok step in particular is a bit harder to find, but I found one at a Big 5 Sporting Goods, which is the big chain out west. Depends on where you live and what stores are near you.