Hey Van Jones,
you could simply buy it here. Installing it goes this way.
With the original 65 mm mount, the spindle will not clear the stepper so you have limitations on spindle length and/or spindle position in the mount. With the optional 80 mm spindle mount , the spindle clears the stepper so you don’t have this limitation.
Note that if you insist on using a 65 mm air-cooled spindle (e.g. like this one), there is a solution to lift the stepper cage so that the spindle fits then and eliminates the “bumps the stepper” problem.