Any countertop supplier will order size you need or cut the size you need?

Look at Bertch bath cabinets and put together a configuration that works for your plumbing. Then, like HIWE said - the counter could be fabricated.

Have you checked out custom sometimes you will be surprised at the price and IMO made to fit is the ideal . I am not sure how the counter came into play right now.If budget is tight do 2 48" ones sxs I have done that many times and then one counter to span both

That turned out nice badgergal

Any local kitchen cabinet showroom can create anything you want out of components. You cannot get a 96" vanity down the hall and into the bathroom without it being in multiple pieces that are leveled and joined in the field.

I have remodeled multiple bathrooms using cabinets from a local kitchen cabinet builder as badgergal and Minardi said. Even for smaller ones it's a nice way to customize the configuration.

I don't think you'll find a pre-fab vanity that's 8 ft. long. As others have suggested, go to a cabinet dealer and get the components. Sometimes those places have working relationships with slab dealers/fabricators, too. Or they can at least send you in the right direction.