Langmusi in detail
The Public Security Bureau is on the road out of town towards the highway. They're of limited assistance, however, and most issues will need to be dealt with in Hézuò or Línxià.
There's nowhere to change money and no ATMs that accept foreign cards, so get plenty of cash before you arrive in Lángmùsì.
China Post On the main road through town, back towards the highway and away from the bus drop-off.