If you absolutely cannot get version 9.1, consider these legitimate alternatives:
Many universities and large engineering firms host REFPROP on internal servers. If you are a student or employee, check your institution’s software portal or contact your IT department for the .exe installer. Installation and Setup Nist Refprop 9.1 Download
While Version 10 offers better speed and more fluids, Version 9.1 remains popular because: If you absolutely cannot get version 9