Your best bet is to use one of the main online ticket places like expedia, priceline, cheapseats.com and etc. They get major contracts with airlines and sell with the same mark ups if not even lower then most of the travel agencies since they sell so many tickets. Buying tickets in advance (unless its under 14 days) doesn't really matter, if there is space there is a good price, no space your shit out of luck.
-People say marijuana ruins your life, I just say I take the scenic route-