Green’s Dictionary of Slang

an informer.

1924		S. Scott Human Side of Crook and Convict Life 176: He was not liked very well by the other lags, he was known as a ‘shopper’ (a warder’s tale-bearer).
1965		(con. 1940s) B. Behan Confessions 87: The Irish, for all their peculiarities, are not a nation of shoppers.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 166/1: shopper n. an informer.