Please note that as of Feb 20, 2014, not all realms are able to find crests in the wild yet.
Once you you have your Great Dragon armored and capable of attacking, you can start attacking level 6 and higher Forests in hopes of getting Nightshade Dragon crests as a random drop.
You must already occupy your maximum number of wilds: if your attack occupies the wild, you won't get a random drop. Each attack must be accompanied by a dragon. Crest drops generally consist of 1-6 crests, with a one-hour cooldown after every drop. Six crests are aboe to be obtained each day.