Plenty of bar bags have map pockets, but apart from Ortlieb's (which will display twice as much map as this map holder) they tend to be pretty small.
This map holder from Rixen Kaul has a pocket that's 24.5 x 12.5cm, which is big enough for one section of a folded OS Landranger, and the fit is even better if you remove the cardboard cover first. The transparent PVC pocket is fastened underneath with Velcro, and will keep rain and wind from ruining the map. Unusually, you can also rotate the map window through 360 degrees at 90 degree intervals. The map holder itself quick-releases, leaving just the Klickfix Mini Adapter on the bike for those times that you don't want a map. This Mini Adapter will fit to any width of stem or handlebar and is compatible with other Klickfix products, such as tool bags. It's not quite as elegant as Rixen Kaul's similarly priced Mini Map Holder, which is made of folded Perspex (and is compatible with the Mini Adapter), although weather protection is far better.