First of all, congratulations on your nomination. My son was up for one of the awards a few years and had a good time at the event. As a male, it was a lot easier for him just to pull out a suit and tie!
I'd second the J Crew suggestion if you are looking online; most of their dressy wear is geared to weddings, but some of the dresses are quite wearable for other events.
I have a couple of Calgary suggestions, though. First, if you are up for going through a mall, I'd suggest you check out Chinook Mall on Macleod Trail. The section near the future location of Nordstroms has some nice shopping including BCBG, Armani Exchange, Burberry, Judith and Charles, etc. This location is supposed to eventually also house J Crew, Eileen Fisher, and some of the other US retailers.
Another suggestion is to do what my DIL does and call a personal shopper at Holt Renfrew, located in the city centre, to set aside a few items in your size and price range for you to check out. The Calgary contact number is (403) 269-7341. If your time is limited, that might be a better option than wandering through a mall trying to find something in your size. Holt can be expensive, but, some of their house brands are quite reasonable. For example, their Calgary store has the following peplum top and matching color-blocked skirt that could be interesting. And the price is quite reasonable.
http://www.holtrenfrew.com/sho.....0-9001-009http://www.holtrenfrew.com/sho.....0-9003-009