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You can download the full RFA.net SDK package from the RFA.NET downloads section.
Once you unzip the contents, you can find the required DLL in the Libs\WIN_64_VS120 subfolder.
RFA.NET cometability with .Net Core
Create OMMItemEvent and Data.FieldList - RFA.NET
RFA.NET RequestToken question
RFA.NET - Session.Acquire gives timeout exception when called from server side