I think you may be hard pressed for an all white material. Natural materials that are predominately white, will still have undertones that contrast the white

I can only think of tile. You would have to apply it to a pre-cast material to create a coping stone.Large sizes are available.

There are custom pre-cast companies out there that can make virtually any color.

True white in any material, may be more susceptible to show any stains from dirt, juices, wines, etc....

Thank you for your response. I did find a sandblasted marble which I'm hoping by sealing it might help from the elements. Do you have any such experience with such material? Do you think that might work?

I don't have any experience with marble. I'm in the Northeast and a concern with marble to me in the temp swings, is any issues with moisture getting into the fine natural fissures in the material, possibly causing damage. I don't know your location.

Granite is another consideration, but it may have not be a true white. It's nice material but a bit pricey.

Sealers can sometimes change color appearance after applied. I would try a test piece and let it dry before moving forward on sealing the material installed.

https://www.federalstone.com/

Have used these recently, very white and high quality.

Yup, Federal Stone a consideration. We used them for a large commercial project several years ago. They're mostly geared for commercial/public use, but they do custom orders and they may have something to your liking.