Garage Door Rollers - Whats The Difference ?

[@ScottDLR]

Is the "Z" bearing roller sealed? If not then it isn't the best.

[@DennisJohnsonDrummer]

Is the porice of the "Z" rollers $39 each or for a set?

[@marcm4452]

I'm having a hard time looking online to replace my 2 inch x 8 inch stem rollers. Know of any sites?