I was there about 4 years ago. Girls can be picked up at discos; there is a street that is full of these discos. Unfortunately can't remember the name of the street but it is down town. Think the price is about USD 20-30 for overnight. The problem is that these places are really dark and you can't get a good look at the girls. Also many do not speak Englsh.
Anyway check out this forum. Seems to have some up to date info:
InternationalSexGuide - Rangoon