http://www.thebedshop.com/linen_custom.phpThere was a posting a little while ago from someone who had set up in business making such covers but as S L says a local seamstress could easily modify an existing duvet cover etc.
We have a 2in sponge mattress overlay which we put inside a duvet cover as the bottom sheet.
The excess from the cover is folded underneath and pinned back with safety pins
The top duvet folds under along the two sides anyway and looks quite neat when made up.