Hi mugtastic,
Their website was still up at the end of last year although the promotion was out of date (see the wayback machine here) but it’s down now and their number is disconnected as you mentioned so I’ve removed them from the Toronto list. Thanks for the heads up 
The only reliable way to know with any certainty would be based on your own personal experience because there are too many unknowns, variables, and personal preferences involved for anyone to be able to make a specific suggestion based on “theory” or "specs (either yours or the mattress if you knew them) with the limited information you’ve provided but post #2 here and the topper guidelines it links to can help you use your actual sleeping experience as a reference point and guideline for choosing the type, thickness, and firmness for a topper that would have the best chance of success.
A good “average” choice would be 2" which would provide a “little to a fair bit” of additional softness and pressure relief. 1" would be more in the range of “a touch to a little” which may not be enough additional softness or pressure relief and would be more for “feel”. Choosing a 1" topper can also be a good strategy because you can also add an additional inch if you need it although the cost would be a little higher so the “ideal” would be to make the “best” choice the first time.
When you are uncertain about which choice would be best for you then the most effective approach is generally a more detailed phone conversation with a knowledgeable retailer or supplier since they will know the specifics of their toppers better than anyone and it would also be helpful if you could provide them with the density or ILD of the latex in your mattress, more specific information about any toppers you have used and done well with, and more about your body type and sleeping positions so they would have some kind of reference point for what you consider to be soft or firm and which of their toppers would have the best chance of success.
If you are uncertain about your choice then the exchange or return policy may also be a more important part of your choice just in case the topper you choose doesn’t work out as well as you hoped for.
It may also be worth considering using two separate toppers with the same thickness but a different firmness on each half of the mattress if your needs and preferences are very different.
Phoenix