Printing Industries of America/Graphic Arts Technical Foundation site (both industry trade groups). Most of this appears to be password protected for members-only, but there may be some accessible information.
North American Industrial Classification System (NAICS) information for commercial screen printers. This code is used in many databases as an identifier. Note that these establishments print on many different media.
Trade association devoted to specialty imaging. Link at the bottom left to Surveys & Statistics to view reports and stats that they have compiled. The site in general also includes some information on trends.