Ubuntu touch is available via the ubuntu Hompage / Wiki:
Installation instructions delivered by ubuntu
Neverthe less I would suggest goint with Android / Ubuntu Dualboot first. Instructions found here:
One thing to add for the instructions regarding dual-boot (after installation): If your telephone is turned of it will start Android automatically. Then you have to open you app-menu and start the ubuntu-app. From there you can click a button to shutdown and boot into Ubuntu Touch.
And another thing for installation: For device unlock you have to get usb-debugging on. Therefor you have to click the build-no several times to become a developer. Then go into the new developer menu and turn on usb-debugging. After a restart and completing unlocking the device you will need to repeat that step again!