HSO is no longer available as a plugin. HSO is now the HALion Symphonic Orchestra VST Sound Instrument Set.
You can use the HSO set in HALion Sonic SE, HALion Sonic, and HALion.
HSSE comes free with Cubase, so this is where you would find the “HSO VST SIS”.
Here is where and how you can find it in HSSE, from Marcus from Steinberg, a couple of years back.