What window part do I need for this window?
It's a window latch and keeper. Unscrew a good one, head over the your local hardware store, and see if they have a match in stock. Pay close attention to the distance between the screw holes and the how tight the two parts are pulled together. They come in a lot of styles, e.g. this would work if the keeper was on a horizontal instead of on a vertical surface:



You can also look online, and worst case scenario, you'll have to go to the window manufacturer to get the replacement parts.