I have a king sized duvet on my queen sized bed. I have the measurements of the duvet (and other measurements) in the 'notes' section of my daytimer and if I see a beautiful duvet cover I check the measurements against my notes. Last month I scored a Frette duvet cover (perfect size) at Winners. Still spendy, but much less than at regular retail!
My favourite duvet cover was one I had custom made from two (king sized) flat sheets. The fabric (sheets) were expensive, but the seamstress's fees were completely reasonable, and I got exactly what I wanted!