Rakks are the bat-like creatures that fly in the air and swarm in groups of about 5. You should find swarms in different areas around the game or during the Rakk Hive fight when it shoots them out of its mouth. Simply shoot them as they swoop down and attack. On the quest "Get the Flock Outta Here" there is a hive near the windmills that you can get a lot to spawn.
Another method is to use the Siren class and use the phasewalk action-skill when they're within a close proximity. Because Rakks have fairly low health, they should die as you enter/exit phasewalk. -Thanks to KFZ Scrubs.