I've had a lot of luck at JC Penney for dresses in the 2X, 3X range when I'm in the states.
I shop a lot online - Igigi is a go to for "fancy" dress stuff, and it is pricey, but you can't beat the quality. A more inexpensive option is swakdesigns. There casual wear is great.
Old Navy Canada has started carrying some plus stuff online too.