There's several ways to get a Plottie: You can look for available official or guest adoptables, buy a slot so you can make your own Plottie or check our Transfers Center to see if there are any available Plotties looking for a new home.
You should also watch out for seasonal Free MYO events or festivals and games were adoptables or MYO slots are given out occassionally. To be notified of events like these please become a member of Plottie-Greenhouse on DeviantArt.