Storage Spaces Direct (S2D) is sold by Microsoft as a software-only solution. Server hardware must be acquired separately.
You can find a Hardware Compatibiliy List here: https://www.windowsservercatalog.com/default.aspx
Microsoft recommends deploying Storage Spaces Direct on hardware validated by the Windows Server Software Defined (WSSD) program. For Windows Server 2019, the first wave of WSSD offers will launch in mid-January 2019.