The ISO image is available here:
Experimental: Tribblix 0m16 SPARC (md5 hash: f88bb1752df1b039102e09e421a6e135) - Release Notes
You can upgrade a 0m16 SPARC install to 0m18 - Release Notes.
Seriously, when it says experimental, it is, although Milestone 18 is much better. Tested on a T5140, both physical and in an LDOM, a SunBlade 1500, and a SunBlade 2000; status is "works for me".