[pushkins]

Unless you are a contractor getting a window from anywhere other than Lowes or HD might be difficult.

What you will need measurement wise no matter where you go is the following, you will need to measure the furtherest areas of the window frame. In your pictures you can see the little window stops that allow the panes to slide up and down. You need to measure the opening inside these. When measuring the height you want to measure from the top to the inside edge of the sill.
When you remove the panes and stops all else stays in place and the window (replacement ) screws in place.