Torontonian here.
If you're talking about Mustachian clothes shopping then I would hit the secondhand clothing shops like Value Village or Salvation Army. There are several locations scattered throughout Toronto. If you go to higher end neighbourhoods like Rosedale or Yorkville, you may find better quality stuff.
If you're thinking more along the lines of discount clothing stores (along the lines of Ross Dress For Less or Marshalls) then check out Winners. You will have to dig, and it's hit or miss in terms of whether you find what you want.
If you're looking for good quality brand new brand name clothing, then you can find good outlet shopping in St. Catherine's. I think it's called the Outlet Collection at Niagara, but it's right off Highway 401. It is a bit of a drive from downtown Toronto. My hubby found good work clothes at Eddie Bauer, Haggar, and Banana Republic. Not super cheap, but the quality is good.
13% HST (harmonized sales tax) for new stuff. No tax on secondhand goods. I'm Canadian so don't know much about what taxes are refundable.
However, I must warn you even with the exchange rate you may not find things to be a lot cheaper in Toronto, especially because of our sales tax. Also, the selection may be more limited than what you may find in the states. There's a reason why Canadians like shopping across the border.