So I was never one to really post my own findings on here (Sorry) But I wasn't sure if this was supposed to go in the "Guides" section or the "Exploits" section, since I'm pretty sure this is not working as intended.
There is an abnormally large supply of rare "Grove Vipers" in the Jade Forest, Just battle a random pet you find there and in the second battle pet slot there will usually be a Grove Viper and it will ALWAYS be rare.
I have battled about 6 pets in a row and they have all had rare grove vipers with them, thus making an easy achievement, by the way, if you capture 3 you cannot capture more, so just release it and continue capturing rare snakes.
It will look like this almost every time ^_^ :