No mattresses made in The States, can't be shipped to Canada without a lot of fees, tariffs, and taxes....making them way too expensive.
Happily, we also carry a high quality line of mattresses that are made in Canada.
White Glove Delivery within the US and Canada is free, and without having to charge you any of your national sales tax.
The king of mattress that will hold up best under a 400 lb man is all, natural latex.
Since you find the floor fairly comfortable, I suggest their "Bourdolay Firm" model.
You can always soften it with a softer latex topper, if you later feel it necessary.
Here is a link to Natura's line of all latex mattresses:
Natural Talalay, All Latex Mattresses. Natura uses only the highest quality materials available for their Latex Mattresses….(No Chemicals and No Fillers)…with pricing that is more competitive than many other brands of similar quality.