Getting Naxxramas cards without buying Naxxramas?
Solution 1:
The below answer was true up until the recent introduction of "Standard". Naxxramas (and all other adventures over 2 years old) have been retired, and their reward cards are now craftable.
Previous answer ---
"All of the cards from Curse of Naxxramas can only be obtained by defeating the encounters found within Naxxramas."
-Aratil, Blizzard Community Manager
Thread found here.