NitrOS-9 current version is 3.02.08 (May 2008)
Get NitrOS-9 Level 1 (For the Color Computer 1, 2 and Dragon 64)
Get NitrOS-9 Level 2 (For the Color Computer 3)
Get NitrOS-9 Level 2 (For the 6309 equipped Color Computer 3)
Or look for older releases in
Source Code
You can get NitrOS-9's source code from SourceForge's CVS Repository