All? There's a plethora out there in the $4-8 area - not from the traditional us based Mac accessory companies, but probably from the same factories in China. All we're talking about here is a sheet of plastic cut into the shape of an iPad. I doubt these accessory companies have material engineers on staff and own intellectual property related to these products.
We have the anti-glare static film and the clear static film, and then we have the clear adhesive stuff which likely wasn't intended as an optical coating in the first place. These materials were likely designed for other purposes initially (military, space, etc) Is there any evidence out there that you aren't getting the same thing regardless of supplier?