From Wiki

Marmite is readily available in Canadian supermarkets, although they dont always know what it is, or where to stack it.

It can be placed with the spreads, or the baking goods (yeast content) or the marinades. But its there, somewhere. probably.

Usually its only available in small 125g jars, and it costs around $4.50

Larger jars may be found in specialised import food stores, but in general the imported food stores are expensive places to shop at best.

Also worth checking out South African import stores for SA marmite (different coloured label, slightly stronger taste, although that could be due to the length of time in the jar!)