I am looking for reference on CasselmanShalika formula for GL(n) and PGL(n) at finite place p.

In this paper, Shintani proves the CasselmanShalika(Shintani) formula for GL(n). This preceded CasselmanShalika's paper by a few years. Several of Cogdell's expository articles on Lfunctions have easytoread descriptions of the formula. 

