Manor usually have a huge selection of sizes. You can also try Pfister or even the specialist bedding shops, probably a sure bet, but most likely more expensive.
It will, indeed, be very expensive. My wife and I completely shelved our plans for a king-size bed (already shaky after difficulties with the bed itself) when the only full-sized doonas(/duvets) we could find were scarily expensive (even mid-range ones were around CHF1000).
I've come to the conclusion that the Swiss don't sleep in beds bigger than a double and larger sizes are only sold here to rip off the rich foreigners.
Ok found the cataloge, and they have a very nice duvet 240x240 costs 149frcs if you tell me the size of the pillow cases you need then i can give you the price for them
Seems to be some confusion between duvets and duvet covers......a good goose down 4-seasons 240x240 duvet could easily cost up to 1000 CHF, but not the cover...