This is an omnidirectional wheel. If you know what this is, you know that they're a real pain to find, and expensive when found. They're expensive because it's a niche item, not because it's terribly complicated or hard to produce -- an excellent candidate for home replication.

Unlike commercial omniwheels, this comes with a Futaba-style servo mount already in place -- it's designed to be screwed directly to the servo. If this isn't your style, replace the object named "Servo Collar" in the Blender file with the mount of your choice.

v1.1 Update: Based on input from Simon Kirkby, I've enlarged the centers of the roller wheels and changed the axle mounts, which no longer need drilling.

Gather: 8x 12mm lengths of 3mm rod. Slightly smaller rod will also do.

Print:
1x omni-36mm-plate.stl
1x omni-36mm-plate-outer.stl (identical, but without the servo collar)
8x omni-36mm-wheel.stl

Glue: the plate to the outer plate, with the outer wheel mounts offset 45 degrees from the inner.

Mount: each roller wheel on a length of axle. Sand or drill if needed to make it rotate freely.

Glue: each axle in place.