Mountain board wheel sprocket or pulley

It seems I can find any pictures of how to mount the sprocket for the pulley. Where do you guys get the pulley or the sprocket for those Wheels or rim

@Ssmario1 can you specify which wheel you are looking to add a wheel pulley to? There are a few vendors that sell them, but the fit depends on the wheel that you have. - they have lots of #25 chain and sprockets in there - quite inexpensive.

As @treenutter said mounting them depends entirely on the wheel.