I'm unfortunately at a loss in that I've never heard of a MacBeth Booth. My best recommendation (if you haven't already done so) would be to research the websites of the ISO17025 accrediting agencies. Second would be to contact the manufacturer of the MacBeth Booth and ask them if they have any recommended calibration vendors.
Do you know any other names it may go by? Alternatively, can you tell me the manufacturer/model? If you can post that, I can look at the calibrated parameters and perhaps give you some additional input. There are a lot of calibrated instruments whose certifiable parameters are more common than the apparent function of the system.