According to Steve Wind-Mozley, head of online @ Game, the best way of seeding promotional & discount codes online is a) via affiliates (if you are happy to to pay commission and network fees above the cost of the promotion codes and b) via a nice plump email send which should do the trick along with placing it on some money saving sites.
The best place to seed promotion codes though is with your current customers - they already get your proposition and chances are they will know other people who will too.