If I'm copy-and-pasting a definition, I'll just do it the way you do, and simply cite NOAD.
If I want to paste a link to a specific definition, though, I'll usually use Macmillan, Collins, or ODO. Those sites seem to be fairly "clean" and don't have too much heavy advertising. (I avoid M-W and Dictionary.com, because those sites seem more cluttered with advertising; M-W in particular will sometimes contain video ads that start playing after landing on the page.)
If I want to give a link to other dictionary links, I'll generally use OneLook for that. And if I want to use a page that has definitions from more than one dictionary, I'll often use Wordnik.