Finding good reference material can be hard. You're unlikely to find blueprints and such as these documents are kept confidential, but you may be able to source elevation images and diagrams on brochures and specification sheets. Spec sheets are good for main measurements too - check the images against them to ensure they're accurate. If you've got sources, you might be able to get info from those who service or drive the buses.
If you're working from photographs, distortions are going to be your enemy. The photograph needs to be taken as perpendicular as possible and with any lens distortions, etc. corrected. Remember to keep straight lines straight and parallel, even if that means deviating from the photo - try to hit that balance of what the camera says is there and what should actually be there.