Well, I'm guessing you need to contact D-Link and get one from them.  You could probably cobble one together, but I'm not sure it's worth the trouble.  
FWIW, I flipped my power brick over and noticed there is a light on it, I didn't realize that.  It's a dual-voltage supply 5V@3A and 12V@3A, so any replacement would have to have at least those ratings.  In addition, you'd have to use the plug they use to connect to the DNS-323.