A step up snake is often considered to be an irian jaya carpet python or an amazon tree boa.

A great book on the subject is The More Complete Chondro by Greg Maxwell. The Complete Chondro is his first book, and the more complete chondro is basically an updated version. If you're serious about getting one, I'd recommend reading it first.

They have far more delicate spines than a RTB and are not really considered to be a handling snake btw.