Check locally for breeders instead of going to petstores. Pet shops may have an abumdance of birds, but these bird have little to no human attention, which can promote unhealthy or destructive behaviors and mental problems. You are looking for HAND-FED parakeets. These parakeets have the most human attention and are more apt to bond with you versus the parent-raised parakeets that just want to be with other birds rather then people.
Here are some places to look on the 'net:
http://www.aviary.info/modules/myAds/
http://birdmart.com/classifieds/
http://www.babybirds.com/classifieds.html
http://pub42.bravenet.com/classified/show.php?usernum=3558513167&cpv=1/
http://www.postaroo.com
http://www.upatsix.com/classifieds/fs_results.php
...and your local newspaper classifieds. Sometimes your petstores will have a "community board" where privite breeders will advertise they have birds for sale, or people will advertise they need to re-home their birds. Before buying from a pet shop, check all other avenues first. There are also rescue groups you can look at for a parakeet, or ask your local avian veterinarian to refer you to a breeder.