There is no difference between Canadian and USN cuffs on the high-collar white tunic. In fact, there is no cuff per se. The sleeves are hemmed (i.e. turned up and sewn) and this should be included in the cost of purchasing the tunic from the NEX.
Remember that there are two sizes of buttons used on the white tunic, large ones (which are the same size as those on the blue tunic) and smaller ones (same as mess kit buttons, but not as small as the ones on your cap). All these sizes are available at Clothing Stores, but they may have to search for the pocket buttons. Another option is to buy nice shiny ones from a tailor.
As for mess kit, if your on the West Coast, go to the Canex tailor (Mia). She does great work at a reasonable price.