Unfortunately, there's absolutely nothing downtown, you'll have to travel.
I used to work at Duke's Source for Sports in Etobicoke. Guys there are great and they always have good deals and stock. From Union, you would take the TTC subway to Kipling station out west.
Eaton Centre has a Sportchek but stock/knowledge is low.
Just Hockey Source for Sports as well as Majer Hockey are other good places to check out, but you would need a car as they're rather far from you and hard to reach by transit.
There's also Toronto Hockey Repair, which is a small shop located at Dundas West station (same direction of travel as if you were going to Duke's).
Let me know if you'd like any other recommendations!