It says in your website that Ubuntu is a free operating system.
How can I get the source code for "Ubuntu for phones"?
You can follow the instructions for building Ubuntu Touch from source from this wiki page.
Building from source is quite an involved process, involving getting Android drivers and flashing a mobile device, which is why I'm not going to attempt to summarise it here. Just follow the link, it's a wiki so hopefully it will be kept up-to-date.
If you just want to try developing Ubuntu Touch apps on your desktop, you don't need to build Ubuntu Touch from source, you just need the Ubuntu Touch SDK.