As for a free download, I couldn't find any reliable sources that offer Octane Render for Cinema 4D R26 for free. Octane Render is a commercial product, and its developers, OTOY, offer a free trial version, but not a full free download.
: If you specifically need a free, unlimited GPU renderer, Octane offers a Free Tier (Prime) Octane Render Cinema 4d R26 Free Download
While individual users are rarely prosecuted, companies using pirated software face massive fines. OTOY and Maxon actively cooperate with anti-piracy organizations like the Business Software Alliance (BSA). As for a free download, I couldn't find