We ordered ours from Kukunest and we love them.

target (of course). and here's some other sites:

http://www.designpublic.com/shop/kids/kids-bedding

http://pillowsandthrows.com/modern-baby-and-kids-kids-bedding.html

http://www.pokkadots.com/items.cfm

and you could always buy some cute fabric from a site like Purl Soho or ReproDepot and make one! i recently posted a DIy for crib sheets - the same concept applies only larger for twin sheets, I will post a duvet DIY soon...
http://prudentbaby.blogspot.com/2009/09/diy-cute-crib-sheet.html

Serena & Lily has some great twin duvets:
www.serenaandlily.com

IKEA... or make it yourself. It's a very easy sewing project!

I also had a hard time finding one I liked/could afford so I made one. It was my first sewing project and super easy. I used these basic instructions, minus the ties (I used iron-on velcro tabs for the closure):

http://www.marthastewart.com/article/baby-duvet-cover

I like Etsy.com, sometimes you can get custom things made.. here is a link to a pink and orange one.
http://www.etsy.com/view_listing.php?listing_id=10180054

Our house is kinda retro modern and we love Kukunest!

My 3 year old's twin bed is Intergalactic, his room is a happy cheery space theme. The acid yellow/green duvet cover looks fantastic with his bright turquoise blue sky wall color.
http://www.kukunest.com/bedding.php?theme=intergalactic

And we just bought Lantern Festival for the queen bed in my baby girl's room/guest room.
http://www.kukunest.com/bedding.php?theme=lantern
Its a very bright red, looks cute against her purple walls.

I bought only the duvet set and purchased the sheet set and additional pillowcases at Target to save some money. Its the only place I found sheets that matched the clear bright colors of the duvet covers. We used ikea duvets because they are thin, easy to machine wash and inexpensive.

The kukunest duvet covers have held up really well through multiple washings, sometimes 2-3 times a week (son was potty training), haven't faded a single bit.

I also like that they have toddler bed, twin and queen sizes. A lot of kiddie sheets don't come in queen, but our babies room is also the guest room.

My son loves his bed and everyone who walks into his room admires it.

Oh hey! I haven't tried it, but a friend of mine designed her own sheet set for her child's nursery at www.fabricondemand.com
If you're handy, you could make the sheets into a duvet cover.

I might be in the minority but a duvet for a toddler bed sounds like a complete waste of money. I would probably make something or use a crib blanket until he/she moved out of the toddler bed.

We just bought two sets of twin bedding from a sale at Dwell Studio that we like and will hopefully last a while longer.

If you are willing to spend, another vote for Serena & Lily. They also sell fabric if you want something custom-made.

The Company Store has some for $39.

http://www.thecompanystore.com/parent/Bedding Baby Bedding/5004/E1N5X_2/

Take a look at:
www.mybedlinen.com

They have beautifully made appliqued and embroidered duvet cover and pillows for toddlers, and also fitted sheet and duvet cover for babies.

You need toddler bedding, right? Not bedding for a twin bed? I had the same problem and ended up buying a crib comforter from Target: Circo Baby Comforter - ABC's & Dots. They have sheets to match, too.