Where can I discovery an effective and cheap glare filter?

I want to find a filter to vigorous a flat sheet display and my laptop. Are glare filters within pandemic worth purchase? i.e. however efficient are they?

I've also heard that flat panels have built-in anti-glare. Is this true? Is it worth it to buy a device for it, or would it not make more than difference? Do I need to purchase something special for it?

The second issue is price. What price ranges should I beryllium expecting?