Went on a suet quest myself last year. The stuff you buy in supermarkets is usually mixed with flour and so you will need to adjust the recipe (I think Maple Leaf makes suet).
I ended up finding the real stuff at Boucherie de Tours at Atwater Market. I'd call ahead.
Note: in European French, suet is "graisse de rognons", in QC french it's "suif de boeuf".
I still have some Xmas pudding left from last year. But will make some new one this year too (although I am late, because 4 weeks isn't enough to fully mature the pud, so I cheat and steam twice and add extra booze).
OK, here's the scoop according to MIL: "Any good grocery store IGA Loblaw around meat counter in a clear plastic bag. It's white. Half pound bag."
By the sounds of it, if you don't see it in the store, you should ask the butcher. (Or try a butcher shop, also?)
Hope this helps, good luck!