Go to www.nancychandler.net
There is a great map of this market on this site. You might wantg to purchase their map of Bangkok as it will give you a wealth of info about shopping, markets etc.

there are a couple of night markets the biggest is on rama 4..

Pratunam has a night market which is basicly for cheep clothing.

yes patpong has a night market with t shits and crapy souvaniers
(no. im not trying too push anyone into gong too patpong. but you'll see as many women as men in the night market)..

actully along sukunvit and silom thier are stalls open too faily late but most along sukunvit start closing around 9 pm. .