There are a number of hatcheries that sell Muscovy ducklings (and hatching eggs) - I acquired mine locally and hatch my own eggs. While I've never purchased from them, I'm pretty sure I've seen Al's Quackery post here on BYC:
http://alsquackery.weebly.com/muscovy-ducks.html. Also, while this thread is oldish, there are other hatcheries discussed:
https://www.backyardchickens.com/t/624869/where-to-buy-muscovy-ducks.
So far, I've only introduced a Pekin drake to the Muscovy coop (and they just ignored him), but I'll be integrating a Runner and her mixed ducklings soon. I'm not worried about violence ensuing, but (as with any integration), I'll be monitoring the situation to ensure that no one gets hurt. The Runner is pretty small so would probably be squished by a Muscovy drake - about twice her weight - if one attempted to breed with her.
FYI, Muscovy ducks fly very well, so clipping wings is usually needed, and they have powerful wings and clawed feet...they can be very difficult to hold. I'd probably classify them as a breed for "intermediate" duck keepers.