With your version of Cubase I believe you should have Halion Sonic SE and Groove Agent. I started with that version of Cubase and have always had them available. I don’t use the Plugin Manager when selecting which VSTi I want to use. I usually add a new Instrument Track and choose in the drop down on there which instrument I want to use. If you do that it should show you all virtual instruments you have available.
The link that elzouzou posted above is a good site for finding other free virtual instruments and FX plug-ins. If you are looking for something specific you can always search Google for what you want followed by “free vst.” There are a lot of free plug-ins and virtual instruments available online. Hope this helps.