A word about quarantines....
It's not a foolproof method. I did quarantine the birds that brought MG to my flock. The thing about MG is that infected birds, once they are over the initial symptoms, become carriers. So they will spread the disease without showing signs of illness themselves.
A basic quarantine isolates the new birds from your flock for a few weeks. You should always take care of your birds first, then the quarantined birds. (Ideally, you'd disinfect your shoes and change clothes once you're back at the house.)
To take it one step further, after the few weeks are up and the basic quarantine is over and the new birds appear healthy... take one of your birds and put it in with the new birds and wait a few more weeks. This will check for those latent carrier diseases. If your bird remains healthy, then turn everyone out together.
My plan is to avoid buying adult birds altogether.