They have lockers at all four parks. Some are larger than others, but they all cost the same. Last time I used one it was $5.00 for the day, with a $2.00 refund when you turn in the key. You can get into and relock the lockers all day. If you change parks after getting your key deposit back, all you have to do at a different park is show that day's receipt and pay the (refundable) key deposit again.
Locations --
Magic Kingdom -- Under the railroad station at Main Street.
Animal Kingdom -- to the left after going through the turnstiles; they are up from Guest Services.
Studio -- At the "Gaas Station" on your right after the turnstiles.
Epcot -- Go past the camera center near SpaceShip Earth. Turn right as if you are going to the rest rooms, the lockers are before the rest rooms.