Only way to get retail code (and it is just 8th slot gun without other perks like premium, medic kit 2, smoke nades) is by spending 500 G1 credits on gamersfirst.com. The chance is 1/55,000 and they are handing out 4 or so.
Retail boxes and codes are manually expired by K2 in July 2009 like said above. If you use a retail code now, it will simply say "invalid code!", because they don't work anymore