I agree... that's what I did.
I had 6 risers total for my regular step, so I just bought the high step top by itself.
(not that I do step that high.... I had 6 risers to use it as a bench for weight workouts)
You can get them in a lot of places....
I can't remember where I got mine, but I think it was at one of these..
powersystems.com (where I get most of my stuff)
fitness1st.com (where I got my Troy Lite barbell set