Technically, all baby ducks are taken from their parents if they are adopted by humans. It's just a fact of life.
I would never suggest taking a wild duckling from outdoors. This could cause the parent ducks to be upset, the duckling will be extremely hard to tame unless you catch it the day it hatched, and quite honestly, it's just not ethical.
However, there are alternatives. Farm and ranch suppliers like Wheelers, Murdochs, etc. in our area carry chicks, goslings, and yes, ducklings, shipped from professional hatcheries. If you would like to start your own flock, you can order direct from the aforementioned hatcheries, but the minimum order is 24 birds.
Check your local newspaper, as well. I live in a rural area, and people are constantly advertising the sale of animals.
Contacting your local extention office might work, too. They work closely with the 4-H clubs, and often have resources for locating places to purchase all sorts of animals.
As a side note, if this an Easter pet for a child, do not go through with it. Ducks are not good animals for little kids. Too many have been harmed by small hands squeezing too tightly or forgetting to provide food and water. The same goes for rabbits, chicks, etc. Go for stuffed--you'll be better off in the long run.