I've seen them at most Loblaws or Superstores. Most asian markets sell them as well, they've become very available in the past few years. Kadaeya sold them as well, but I've heard they're closing/closed? :( boo.
You can get panko bread crumbs at Superstore on Richmond Road. There is a sushi area closer to the entrance and produce section, which has Asian groceries isolated from the rest of the store's products. They do have an aisle with Chinese food, but the panko isn't in this aisle.
tre