I hope you have already found the solution, if not please use the following command.
TABCMD GET "/datasources/Datasourcename?format=tds" -f "Targetfile.tdsx"
or with full qualified server URL
TABCMD GET "http://ServerName:port/datasources/Datasourcename?format=tds" -f "Targetfile.tdsx"
TABCMD GET with or without additional param format only return tds file (the metadata of data connection), not the tde file or tdsx file (tde plus tds in a zipped file). When we put format=tde, the return string said *** 406 "Not Acceptable"
Hence, right now, there is no way to get the data source (tdsx or tde) that already published to tableau server.
Tableau Server should for the first time provide the way to get (like the way we download the data source from web or tableau desktop) the data source (tde and tdsx) when Data Source (proxy data server) available to Tableau Server.
I wonder if this capability already in Ideas.