There is some discussion about supersonic wind tunnels
in Liepmann and Roshko.
Also look at "High-speed wind tunnel testing"
by Pope and Goin.
Olin Perry Norton
Also look at Shapiro's book on compressible flow.
He has a discussion of diffusers, and gives some
references that look interesting:
Kantrowitz, A. and Donaldson, Coleman duP., Preliminary investigation
of supersonic diffusers, NACA ARR No. L5D20 (1945).
Neumann, E. P. and Lustwerk, F., High-efficiency supersonic
diffusers, Jour. Aero. Sci., Vol. 51, No. 6 (1951), pg. 369.
If I were you, I would try Google (or Google Scholar),
in addition to looking for keywords like "supersonic
diffuser" I would try searching for these authors to find
any works that reference them.
Also, NASA and NACA reports are available on-line,
so I would search those as well.
Olin Perry Norton