It's been a long time since I've dealt with this, but the instances index (which is available for raw instances and Server/ServerArray instances) will show terminated instances:
https://reference.rightscale.com/api1.5/resources/ResourceInstances.html#index
There is even a filter available for state which you could use to only show terminated or stopped instances when you query.
However, I also remember that, depending on the cloud, terminated instances may not show up for very long within the platform, so if you need the data you should probably get it quickly.
I hope that helps!
