DIY or Buy. Put a dog, cat or anything on it with tee shirt transfer paper. Top Photos: Bob the Dog Duvet Cover from Snurk here. Bottom Photos: Ollie the Cat Duvet Cover from Snurk here. Snurk also has an astronaut, princess and other really creative duvets and pillows on their site.
I’ve already posted DIY tees, pillows, leather etc… using ink jet tee shirt transfer paper - so why not use it on sheets (you can also use 2 sheets to make a duvet cover)? Whatever you can photograph you can print on tee shirt transfer paper - it’s that detailed. You could always practice on an old pillowcase or tee. You can see the variety of iron on transfer papers here (no affiliate links).