[shrug] I assume you've already looked at the list of publicly available codes on sites like mousesavers. Beyond that, just keep checking - this is way early to see any real deals on October rentals.
For peace of mind, go ahead and book a car now on the best rate you can find. Then about once a week check again to see of you can get a better rate. You may find that as you get closer to October (perhaps in August) the rates will change - hopefully downward.