Where? Staplehurst, UK
What? Magnetic Shields provides a wide variety of shielding including entire rooms and bespoke shields.
What, exactly? A magnetic shield is not a non-magnetic material, but one that is used to “redirect field lines away from areas we wish to be field-free.” Since the earth possess a magnetic field, very sensitive experiments might want to remove effects of the ambient magnetic field to make minute measurements. That is the case for an emerging technology of brain imaging. But the FAQ and Technical sections of the company’s website explains this better than I would here.