Are there any airlines from Copenhagen to Stockholm that would not show up on the main travel websites that would have cheaper flights?
I think you're limited to SAS or Norwegian Air on that route.
Looking for other options have you thought about the train? It takes a couple of hours longer but you may find a deal. (Plus the Oresund Bridge is pretty cool.)
I completely agree with Peter - the train is nice, but a bit longer. If you're going to fly, I think Norwegian is your best bet. In my experience, Norwegian and SAS are at right about the same standard, so you'll not be losing anything there. I know for Oslo-Copenhagen, Norwegian is often 20-40% less than SAS, especially at off-peak times.