Ultegra CS 6800 Cassette
Googling 'Shimano CS-6800 cassette' should convince you that Ultegra 6800 cassette are still available, even though that groupset has been superseded by R8000.

Any suitably high-end and light 11 speed Shimano compatible (i.e. Shimano and SRAM) cassette that you can find in you desired spread of sprockets will work.

You will need to check that your rear derailleur has the required total capacity (difference in teeth in the cassette + difference in teeth between chainrings), and can handle a 32 tooth large sprocket. If not you will need to swap the derailleur for a compatible one with a longer cage.