These are not too hard to find, not the most common, but some of the
Microtek, Epson, HP models will scan legal size pages - those with a 14 inch
bed instead of 11.7 inches. The specifications specify this.
Also those with ADF (auto document feeder) should do it (even with a 11 inch
bed), because when usng the ADF, the paper is moved past the scanning
sensors, instead of vice versa. That means that page length is simply a
software function then, not a hardware function.