Any rentable lockers in NYC?

I need some advice...

Every so often I go to NYC to attend business meetings, but when the meetings are over I like to wander around and catch the sights. It starts getting annoying though when I have to lug my work briefcase around, and as anyone who carries anything around for any length of time know, whatever you're carrying gets heavier and heavier, the longer you carry it. Plus it gets annoying when you go to a museum and they have to search your briefcase, particularly when you don't even need the briefcase any more.

Now, they have these types of lockers in ski resorts where you can stow your street clothes, pay 75 cents, lock it up, and take the key with you so that you don't need to walk back out to the car. Does anyone know if there are any places in NYC (hopefully close to Grand Central Station but I'll accept anything at this point) that has lockers like this where a guy can stow his briefcase for the day so that he won't have to lug the thing around with him all over the city? I even checked out a bus station at something or other street and didn't see any lockers there, plus, I asked a few friends who work/live there, and they don't know of any either (although they admit they never had the need to find a locker).

HELP!