The AM omniwheel shown contains only a single bearing. You’ll want at least two-bearings constraining something to a shaft – but no more than two.
In this case, since you’re using a dead-axle, I’d find the appropriate bearing to fit into whatever sprocket you’ve got bolted onto your omniwheel. It’ll probably be an R8 flanged ball bearing and is available from AndyMark as well.
The flanges on the ball bearings constrain them axially in only one direction. You’ll want to use spacers or shaft collars on the side of each bearing that faces away from the wheel to constrain them axially in both directions. Be certain that the spacer or shaft collar you use contacts ONLY the inner race of the bearing or you’ll experience additional frictional losses and, perhaps, binding.
Be aware that flangeless ball bearings are not constrained axially at all in this sort of arrangement and can fall “into” the wheel. If that’s the case, you can add a third spacer that lives between the wheel and sprocket, but I’d recommend getting flanged bearings instead.