Apparently Mountain Dew gave these away via their Twitter page. One was awarded each day from August 26, 2009 to September 22, 2009. So you’re looking at around 30 available.
I found these details on how you could enter:
Each day during the Promotion Period, Mountain Dew will Tweet a â€œcode of the dayâ€ message to Followers of mtn_dew on Twitter. Once you receive the code, you may enter by clicking on the link and following the online entry instructions. If you did not receive an email or if you are not a Follower of mtn_dew on Twitter, you may register by visiting Twitter.com, clicking â€œFind on Twitterâ€, typing in â€œmtn_dewâ€ as who youâ€™re searching for and becoming a follower. Once you are a Follower, visit www.MtnDewHalo3ODST.com (â€œWeb Siteâ€) and follow the online registration instructions.