products preserve epis of locales

Where can I find a published proof that if X --->> Y is a (not necessarily regular) epi of locales then so is X x Z --->> Y x Z for any locale Z? NB (I know that) this is not true for general pullbacks of locales! Ideally, I'd like to cite a textbook-style account that begins by characterising epis and regular monos as idempotent adjunctions, saying that they form a factorisation system, and describes the product of locales fairly explicitly. Paul