You can get the source code for green screen extraction through the following methods:
1. Search keywords like "green screen extraction" or "green screen segmentation" on code hosting platforms like GitHub. You can find some open-source projects that include the source code for green screen extraction. You can choose a suitable project based on your needs.
2. Search for tutorials and sample code about green screen extraction on the internet. Some tutorials may provide relevant source code for you to learn and use.
Please note that green screen extraction is a relatively complex image processing task that involves color and edge detection, segmentation techniques, etc. Therefore, you may need some programming and image processing knowledge to understand and use these source codes.