Best thing to do would be to call Forgeline.. if they don't respond on here first.
If they used to have 18" then maybe they can still make them. The stepped lip is stronger, but in our pro touring world that doesn't matter as much. It also depends on your front spacing, and if there is even room for a stepped lip. The stepped lip centers are slightly smaller in diameter, which changes the look. A 19" stepped lip center looks like an 18" flat lip center (SO3P).