The two most popular roadbook holders are from MD and Touratech. The most popular choice being the MD one as it works with rubber bands and urban legend has it that it is more reliable than the Touratech. Most of this opinion is based on Touratech roadbooks from a few years back that had plastic gears that made them prone to failure. The later Touratech roadbooks have metal gears and are very reliable.
Both are available on the the web for purchase, google them :
The roadbook holder discussion has happened a few times on ADVRider, search for the threads as some also discuss the Free-To-Ride roadbook.