Ubuntu Touch is no longer maintained as a core product by Canonical. However, the Ubports community are continuing development.
A PPA has been created to contain 3rd party applications designed for Ubuntu Touch. The list of applications considered for inclusion can be found on the wiki at Touch/Collection. Applications in this PPA are currently being manually curated, built and published by members of the Ubuntu Touch team. Contact popey or mhall119 if you have any questions.
The PPA is not enabled by default on the Ubuntu Touch preview images, but can easily be added via ssh to the device using the instructions on the page linked below.
If you are a developer and would like your application included in this collection, please add a link to the above wiki page. If you already have packaging skills, please consider adding packaging to your branch before linking, if not, don't worry, the Ubuntu Touch team can help with this.
See also: CoreApps PPA