I travel quite a bit and have used expedia, Travelocity, etc, as well as travel agents. My experience is that travel agents can't get cheaper flights like they used to and expedia/Travelocity almost always show the same prices (and sometimes cheaper) than the airlines themselves.
I've heard of people getting cheaper flights on cheaptickets.com or kayak.com, but I never have. Also, a lot of times when people get really cheap tickets it's because they book their flight last minute, which obviously isn't what you want to do.
If money is really tight, you should consider driving. I drove to Disney two years ago (from Mass) with three kids (2, 4 and 5) and it wasn't bad. The drive home sucked, but it was worth the money we saved in flights.