It looks like your'e going to struggle finding 1/18th scale figures that are not Napoleonic soldiers and cast in white metal. But the Ultimate Soldiers range does offer the possibility of conversion. If you dismantled one of their US Army figures and modified the torso, arms and legs to match British battledress then took moulds from those parts. You could cast any number of basic figures. The heads might be slightly more difficult as Canadian wore both the classic "Brodie" helmet and the newer Mk3 "Turtle" helmet. Webbing and belts were the British 1937 pattern.
Firearms for Canadian troops could be either British or American, Some unit being equipped with the M1 Garand instead of the Lee Enfield.
It's a bit of a performance but repeat casting means you only have to modify one figure.