One of my favorite stops at St. Lawrence Market, Kozlik's has great mustard. Whenever I am there, I stop by regardless of the crowd. And since they always have pretzels to dip into the mustards, there is always a crowd. (FREE FOOD!)
Amazing varieties of mustard, and they are willing to let you try them all. I always find something new. The latest is the balsamic mustard. Yum. They have some pretty spicy ones, and some sweet. It's a good condiment, and Kozliks makes the best.