I download the installer for windows. Where I can download the source of this installer, I want to add some actions.
I don't think you have access to the installer source code
I think you can install Alfresco and extended it adding new actions.
For example, you can check this tutorial:
Creating Custom Actions in Alfresco | ECMArchitect | Alfresco Developer Tutorials
I want to add actions to the installer, for example I want to add Alvex extention. So after installation I have to add Alvex files to alfresco directories.
That's right. After installing Alfresco you can install add-ons. But, you can check out that this extensions are compatible with your Alfresco version.
But I want to add some function to Alfresco Installer, that should add alvex files to alfresco directories. That's why I need installer source code. And I don't know, how I can get it.
You can create your own shell script, that invokes a text based Alfresco installation with the bundle installer (and some predefined values). And then add your alvex files or whatever over the created structure.
$ ./alfresco-community-installer-201707-linux-x64.bin --mode unattended --alfresco_admin_password secret --alfresco_ftp_port 2121 --prefix /opt/alfresco52g
Retrieving data ...