Al booking sites use the same airports and airlines. There isn't a second set that doesn't charge as much. Prices fluctuate all the time, so really, the only thing you can do is check daily until you see the fare you want.
I needed round trip from DC to Boston. I could go really cheap, but I won't fly Spirit. I've been tracking the costs for a couple of months and they haven't budged. Yesterday, they jumped $40 pp, this morning they came back down again so I booked. I doubt they get much lower, but you never know.