Weaver, among others, make them - usually can find them at Canadian Tire or Amazon.ca. Likely you are looking for a "full length scope mount base" - not sure about the Picatinny part, though - I think Weaver bases will accept Picatinny rings - will not be as many slots though - most of which you will never use. To be 0 MOA, it needs both ends the same thickness - if they are 4 inches long, and "flat" on both the top and bottom, than the front end will be circa 0.022 inches thinner than the rear, to be a 20 MOA rail - if 1" is 1 MOA at 100 yards (it is not, precisely, but close enough for most people). Pairs of rings are normally identical height - with a very few exceptions (Ruger 77 centre fire, for example).