Hi ellochka99,
For the most part they certainly do … unfortunately. The ones that don’t (and don’t start rolling their eyes or telling you that only engineers need that type of information) are the ones that tend to have some knowledge about mattresses 
The only actual “recommendations” I make are the members of this site but of course there are many others across Canada and the US that I think highly of, are also knowledgeable, and that I would be comfortable dealing with. There is a list of some online sources that ship across Canada in post #21 here
Post #3 here includes the better options and possibilities I’m aware of in the Halifax / Dartmouth area.
Memory foam is too soft to be used as a support layer and always needs a firmer support layer underneath it (such as firmer polyfoam, latex, or innersprings). It can be used as a comfort layer or as a transition layer (in the middle area of the mattress) but as a support layer it is too soft and also continues to soften with time over the course of the night.
A forum search on Zedbed (you can just click this) will bring up more information about them … and they are one of the few mattress manufacturers that actually pours their own foam.
Phoenix