Where to buy mussels?

Since moving back to Winnipeg a few months ago, I've had a hard time finding mussels. We always bought the Canadian Cove 2 pack of mussels from Costco in Calgary, but I haven't seen them in our local stores lately.

We tried some from Superstore, but more then half of them ended up being dead by the time we cooked them. I live out of town so its not a quick fix to run to the store and get more.

Does anyone have any suggestions on where I can buy good quality fresh mussels? I have never purchased them frozen... Thanks for your help!