You can try:
Cake Creations in Canmore
Wildflour Bakery in Banff
or Fiddleheads Catering in Canmore
If your hotel has a restaurant; the chef may create a cake for you as well. Both Fairmont Hotels in Banff and Lake Louise will create a cake for you.
Good Luck with your search.
Get Married in Banff
As Angie mentioned above, I can highly recommend Barb at Cake Creations.
We used Cake Creations. It was awesome. She can deliver the cake to your location for an extra charge, which is what we did.
Click on each tool to see a sample.