In my experience, a supermarket is also willing to order items for a customer. I have had to ask a meat manager to order duck breasts and other meats. They do it with no problem.
Supermarkets generally want you to take what's in the case. However, I firmly believe in the rule of, "If you don't see what you want, ask for it." (thanks to Julia Child). I generally will get what I ask for.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan