non-merchandisey duvet cover for cotbed - help?

[TaurielTest]

DS has just gone into a cotbed and I have bought a duvet for him (UK standard 120x150) for when it's too cool for just a blanket. It seems very hard to find any duvet covers online or in shops that don't have Thomas / Peppa / Ben 10 etc on.
Can anyone suggest a source for either plain colours or something a bit less branded?

jojomamanbebe
mothercare
john lewis
aspace

Dunelm do some i think

Bus stop
Farm animals
Pirates
Space
Noah's Ark

Or try eBay -- I got a nice duvet cover there newly handmade from a bright dinosaur print fabric for DS from a woman who specialised in making and selling them (I don't remember her seller name, unfortunately, as it was a while ago).

Ooh thank you, off to have a good look :)