I had trouble finding this info when I first got the ROK Espresso Maker, so I’m posting this here for others to find.

The tamper size you want for the ROK Espresso Maker is 49mm.

Why not just use the little plastic one that came in the box?
The one that came with the ROK is significantly smaller than the portafilter, so it pushes grounds up the sides when you push down on it. It has no heft, so it is left up to you to push evenly. This leads to uneven pressure, which can lead to channeling. Plus, regular tampers are just plain cool. Unnecessary, sure, but cool.

Here are a few options for all price ranges and styles:

RSVP Stainless Steel Espresso Tamper, 49mm. $14.95 at time of writing.

 

Zoie + Chloe Stainless Steel Tamper, 49mm. $17.99 at time of writing. (This is the one I’m currently using. See photo above.)

49mm Stainless Steel Tamper. $21.79 at time of writing.

Jimei Calibrated Coffee Tamper, 49mm. $37.00 at time of writing. Clicks when you reach 30lbs of force.

 

Espro Calibrated Convex Tamper, 49mm. $113.67 at time of writing. This cool but expensive contraption clicks after you reach 30lbs of force.