Down east in NS, Hnatuiks sells Challenger slugs (confirmed). I am sure other Ontario/Quebec sports stores sell the Challenger, but I have not spent enough time looking for them in most of these eastern sporting goods stores.
IIRC Accuracy Plus in Peterbourgh sells Brenneke Rottweil Classic slug.
Moving west now, WSS in Saskatoon (unconfirmed they sell) WSS in Edmonton (confirmed) also have Brenneke slugs sometimes in stock. Italian Sports store in Vancouver sells the Brenneke too. TSE Sports in Calgary is presently in the long drawn out process, (government b/s) to import Brenneke slugs from Brenneke USA. Part two question, I think most hunters/users prefer the legal option of a rifle for hunting big game if they have the choice, and yes and maybe yes, but I think it's a mostly a question of economics. (most Canadians, unless hunting critters that can hurt you back, choose fosters slugs as thier first option, or go with a big enough/powerful enough rifle otherwise) Whether we like it or not, fact is most decent hunting rifle cartidges, easily outclass/out range any smoothbore shotgun slug, and are therefore the top choice, if the hunter/user has the option to choose. And $$$....fosters are cheap!
Brenneke/Challenger slug market is a very small for most provinces, other than our north country.
Mostly for wilderness wanderers or outdoor persons, pursuing something other than hunting.
(IE-geologists, forestry workers, prospectors, etc etc etc)