I have gotten birds from Petco and PetSmart, and the birds are young and healthy when you get them. They have their own vets and the birds are guaranteed to be healthy or you get your money back or an exchange.
I have had quite a few parakeets that I got as rescues, but there is no guarantee on these, and you need to be careful. However, once you learn enough about the care and feeding of parakeets, they will reward you with song, speech and tricks!
Think about it! They do fine if left by themselves overnight, or even as long as a week, because they are foragers. Just make sure they have plenty of food and water and they will care for themselves while you are gone. I promise!