Slide locks, at the Home Depot. They're in Aisle 11. They have several different varieties, sizes and colors to fit any and all door that you might need to keep shut.
Incidentally Home Depot also stocks screws and screw drivers which can be used to install these magical slide locks.
Home Depot also has "knowledgeable" employees who will gladly tell you how to install a slide lock if you are unsure of the process.
...all of the stalls in the women's restroom have broken locks.