just a few short years ago, they were ungodly in price. They are down to the $250-$400 per pair range now, as many breeders now have them.
There are numerous sites where these types of birds are sold, as well as 2 gamebird publications with these types for sale
the Gamebird Breeders and Conservationist Gazette, and the A.P.W.S are the 2 biggest
Also a forum dedicated to these as well, but I dont need to post a link here, PM me if you want it.
also Mallard Lane Farms sells them, that's their website as well, though most will be sold out by now on them. You need to find a good reputable breeder and get an order in mid summer in most cases.
Good luck, they are beautiful for a mutation!