The ISO images are available here:
Milestone m20.5, standard image
Milestone m20.5, minimal image
You can upgrade a 0m16 SPARC install to 0m18 - Release Notes.
You can upgrade a 0m18 SPARC install to 0m20 - Release Notes.
The SPARC version is experimental, although Milestone 20 is vastly improved. Tested on a T5140, both physical and in an LDOM, a SunBlade 1500, and a SunBlade 2000; status is "works for me".
You don't need to download the ISO, you can run Tribblix in the cloud.
Some older images are also available.