https://canadasauce.ca/en/collections/tous-nos-produits/products/moutarde

We like Kozlik's a lot. They're in Toronto and in a lot of farmer's markets. Their Market Mustard is probably the closest to yellow deli style

I haven’t really looked for a Canadian replacement to be honest. We have 4-5 different types of mustard in the fridge at all times. I just switched to a Germany mustard that we already use. Here is a link: https://www.londondrugs.com/products/kuhne-mustard-in-a-beer-mug-250ml/p/L1430185 The Kühne brand seems to be available in most supermarkets here in Alberta. There are better brands in specialty stores but this one does it as a daily for me.