Fabricators that have remnants will often cut them to a template that your contractor would supply, with him also installing.

But I’d rethink a permanent bench in a bath so small that you have to do a pedestal sink instead of a vanity. Sounds like you really need a smaller shower and a linen tower rather than an overly long shower. You can still get a 36” long shower and have linen and product storage too.

As posted a remnant can easily be found - at least in my area where stone yards are accessible to the public. Or whoever you are using for construction would undoubtedly have contacts.

If you want a bench get a built in bench. I also have a smaller bathroom and am converting the tub/shower combo to a larger shower WITH a bench.

People who recommend a movable bench never consider where the hell you put the bench when you aren't using it. So it's always in the shower anyway and it takes up more room than a bench would be and it's not nearly as functional.

I know this because I had hip surgery and was supposed to have a seat in the shower for awhile so I had a movable seat. No way would I have wanted to deal with that on a long term permanent basis. It creates clutter - either in the shower or in the bathroom because if it's not IN the shower, you would want to have it close by to put it in when needed. When IN the shower, you then have another object to clean around.

Is there some reason you are going with a pedestal sink? Beside there not having storage below, you are also depriving yourself of valuable counter space. I would only put a pedestal sink in a powder room where there is no possibility of anyone EVER wanting to actually have things on the counter.