There are some in the picnic area to the left of DL's gates, outside the esplanade (before you go through security on the DTD side).
Inside DL it's off to the right from Main Street, past the fruit cart as you go down the street.
In DCA the locker area is to the right after you enter, near the bathrooms and stroller rental.
They have small to extra Jumbo...the biggest ones are in the picnic area, and they were useful for keeping my luggage when I did a day trip on the way from San Diego to the airport and needed to stash my stuff but not take it through security. Those were 12 or 15 bucks.
The smallest are something like $7.