From what I've read, the Radius Rocket won't fit in a IIsi. The Stage II Radius Rocket might fit if you swap the HDD with something smaller. Apparently it's a space issue.

I avoided the Stage IIs because you can only use them as host machines with RocketShare, and not as accelerators.

And I believe the Radius Rockets are still relatively rare, but the economy is not doing so well. I think people are offloading a lot of their more valuable gear at the moment.