To download an iso of the latest Ubuntu daily build then go to http://cdimage.ubuntu.com/ubuntu/daily-live/current/.
There are also some packages on Ubuntu that make this sort of testing easier:
testdrive – run the daily Ubuntu ISO in a virtual machine (meta-package)
testdrive-cli – run the daily Ubuntu ISO in a virtual machine (command line)
testdrive-common – run the daily Ubuntu ISO in a virtual machine (common files)
testdrive-gtk – run the daily Ubuntu ISO in a virtual machine (GTK Front-end)