In the Westinghouse Distribution Systems book, there is a table of typical impedances of distribution transformers of standardized design. It has %Z & %R.
In Beeman's book, "Industrial Power Systems Handbook", there is a table of approximate resistance, reactance, and impedance of single phase distribution transformers. This has %R, %X,& %Z.
Also, I don't have it handy but the ANSI standard on transformers may have some tables in it. I believe it is ANSI C37.010.