Not that there is a super easy way to get one, but i have found the easiest listed below:
Oracle Rep : Once you get to revered with the oracles, you can purchase a Mysterious Egg. Buy the egg, wait 7 days, and click on it. You will get either one of 4 vanity pets (Cobra Hatchling, Proto-Drake Whelp, Tickbird Hatchling, White Tickbird Hatchling), Aged Yolk or you have a chance to get the Green Proto Drake.
Camp out for the Time Lost Proto Drake. You could be lucky and find this one 1st time, but probably not. Apparently its on a six hour spawn, and if there is no one there to kill it, NPC's will take care of that for you unfortunately. The time lost Proto Drake spawns in Storm Peaks at (29, 65).
Do a LOT of PVE and get the Glory of the Raider achievement, which rewards the Plagued Proto Drake.
This is definitely not an easy way... But you could complete all of the seasonal achievements. Winter veil, Hallows End, etc. This is at least a 12 month commitment though, unless you have already been doing them ;)