I would build a nice pen and take your time doing it, plan it well and such, and wait till the fall to purchase your birds, they will be in abundance then and cheaper then they are now. If you are in dire need to buy some now, check Eggbid.com and look under ducks and then ornamental waterfowl, someone is usually selling something!!
The biggest thing about purchasing ducks is the shipping, its almost as much as the cost of the birds, so if you can find something locally, it would be better!!
I would also buy from a private breeder, some of the hatcheries charge like $100 a pair plus shipping, and make sure whoever you get them from gives you the proper paperwork and they are federally licensed, the 3-186 form.
Goodluck!!................Jason