from where can I get the easy installation script for V11?
I believe running the install.sh script with the options --frappe-branch staging & --erpnext-branch staging will result into a v11 installation.
This is not the stable version yet though (which comes from the master branch), so you in case you want to create a production system you will have to switch to the master branch once v11 has been officially released. This is not super difficult but possibly you will run into to some issues doing so which have to be resolved.