Vin -
You shoud be able to get 18" covers in at least 3/8", 1/2", and 3/4" naps... my local Lowe's carries the Wooster 18"ers in both 1/2" and 3/4".
Yep, 18" rigs will really build those shoulders when rolling or backrolling ceilings...
And I love those Wooster lidded trough-style buckets! Once I got my hands on one, I chucked my 18" trays forever.
Oh, and a tip for y'all on 18" rigs - avoid the frames made entirely of plastic... you'd be surprised to see just how much paint a fully loaded 18" 3/4" sleeve can splatter through a room when one of those damn plastic holders break.
My personal fav 18" rig is the Wooster Wide Boy - the plastic body on that one had never broken on me, and the metal adjustable-width holders can withstand a whole lot of abuse & pressure.