Bazaar nightly Ubuntu packages are available from ppa:bzr/daily. The old ppa, ppa:bzr-nightly-ppa/ppa was out of date and has just been deleted.

To get nightly bzr builds on Ubuntu, say:

sudo add-apt-repository ppa:bzr/daily
sudo apt-get update
sudo apt-get install bzr

and as of today you should get a 2.4 beta snapshot.