Look in the gunsmithing section 9/29/2011 and look up the topic lee enfield no1 mk III sight adjustment.Bigedp51 posted a chart that explains the relation of blade height to poi.Once you figure out your present blade height then you can make a educated guess as to the proper sized blade.As to locating a blade online your best bet is to watch e-bay, sometimes a set of blades comes up and then you can just swap them out until you get the result you want.