I'm one of those in the marketing department and there's a compelling case to be made for making sites accessible. If you're running into obstacles from management or marketing it's likely because they don't have a grasp of the numbers.

Getting the data is easier nowadays than in the past. E.g. https://www.census.gov/newsroom/releases/archives/miscellane... and http://www.pewresearch.org/fact-tank/2017/04/07/disabled-ame...