First of all, you need a "de-clicked" lens with an iris gear like this Rokinon 24mm f/1.5 Cine Lens
and of course a stepper like this this Berkey focus motor with a rail mount and worm gear reducer.
Here's the other parts you need to attach the stepper to an eMotimo TB3
A, 15mm single Rod Clamp
Some 15 mm rod, 2pcs Black Aluminum Alloy 15mm Rod - 15cm 6 Inch
A plate to hold the clamp and rod, Desmond 5.5-Inch 140mm 140 QR Lens Plate Quick Release Arca Swiss Compatible PC-140 DPC140
A clamp to hold the camera, Desmond DAC-02 60mm Arca Clamp
Modify the long plate by drilling a 1/4" diameter clearance hole