I don't. I would do the google image thing, but I would refer to live birds photos. Search EJPeiker. He has a section on just birds. Great head shots. Most ducks are loafing, but they are fantastic photos.
Over on Taxidermy.net the member "Seaduck" sell reference CD's. You can buy a specific species or "flying" waterfowl. I have 4 of them and they are useful by seeing multiple shots of bird from different angles. He has a wigeon CD listed under the For Sale forum. He has TONS of options.
The main problem with waterfowl books are seeing the different angles other than a side view.....and the majority of pic's are of standing birds.