Where to find modern comforters?  

Modern Bedding is not so easy to find, I've spent time looking, though
Here's my sources:
www.roomandboard.comwww.westelm.comwww.relishstyle.comwww.modlivin.comwww.chiasso.comwww.dwr.com (very pricy)www.cb2.comwww.pillowsandthrows.comwww.abchome.comwww.designpublic.com
You can also consider IKEA or Target. If you want a comforter but you can only find a duvet cover that you like, get an inexpensive comforter in white and permanently insert it into the duvet cover. If you can sew on a button you can manage to anchor the corners and close the open edge.

Thanks!
Thanks for all the ideas! I have seen so many duvet covers that I liked and I think i'll go with your idea of making it permanent.