I am adding a neutral bar to an existing sub panel and need to insulate the mounting screws so I don't ground my neutral bar, any suggestions on what to use?
You should buy an "isolated bar" or an "insulated bar" to install in the box. This will have insulated mounting hardware of some form (for example, insulated housing for the screw penetrations) which is actually rated for the purpose you intended to put it.
You should probably buy a product from the same manufacturer as the panel you are installing into but here's an example of what you want.
It's not your job to insulate the mounting screws. It's the job of the company supplying the bar, and they must get it UL listed for the application.
Usually, panels are supplied with a neutral bar, and they sell you extra ground bars. In that case you use the built-in bar as the neutral and the accessory bar as the ground.