Where to buy a quality Duvet?

Your title says quality, but then say 'firm' is your style.

However with duvets, quality is often synonymous with lightness.

In my limited experience of duvets, Hungarian goose down is the best, but a less expensive 100% duck down will be almost as good.

I'd steer clear of anything artificial or with feathers. I understand that some people like a bit of weight above them to help them sleep, but that's the role of the bed partner, not the duvet.