Wyverns deal greater poison, which is not fun unless you kill them with earthies/EVs.
The best spawn of Wyvern is ... wyvern island in the Lost Lands, north of Papua in the sea.

Drakes are piece of cake, though slightly tougher than sea serpents. Bad news is, no spawn of Drakes only. Either destard and you'll meet the Greater Dragons ;-) Or Valor forest and you'll meet paragon wyverns and drakes. Or some of the other Dragon spawns.

If someone know any easy spot... I'm eager to know them! It'd be good info to post on my "how-to-make-gold" article