Most places I have seen require that you order 25 ducklings in one batch, but Metzer http://www.metzerfarms.com/ has the requirement that you order 10.
Metzer farms is the only hatchery i know of that lets you order the lowest amount of ducklings (2) without warmer chicks of bonus ducklings... I've ordered 2 ducklings from them two seperate times, each time it costs about 55-60 bucks (There is a heat pack fee, a 30$ minimum, and some other fees i think).
I also ordered from McMurray and didn't have to pay any extra for their minimum of 10 ducklings. I split the order with a friend so we got 5 each. http://www.McMurrayHatchery.com