If you go through a vet they might charge $100+ per bird. If there are any bird clubs in your area it's possible they might do take the sample for a small fee - my local bird club will take blood from a toenail for a $5 donation. A pet shop that focuses on birds and offers bird grooming services could probably do it too. It's not hard to take the sample yourself, but a lot of us don't have the heart to pluck feathers or intentionally clip a toenail too short.
Avian Biotech is the most popular testing lab but lately my anti-virus software doesn't like their website so something might be wrong there. This is the lab that my local bird club recommends:
http://www.petdnaservicesaz.com/avian-testing/ It's in Arizona and they recommend it to support a local business, not because it's any better than the other labs.