Yes, a special trekking permit and ACAP are required to enter Upper Mustang, and these permits can be obtained through Nepal Sanctuary Treks.
Affiliations & Recommendations
Adventure Travel Trade Association
Nepal Sactuary Treks - Day Trip
Government Of Nepal
Nepal Tourism Board
Pacific Asia Travel Association
Nepal Mountaineering Association
Nepal Association of Travel and Tour Agents
Trekking Agencies' Association of Nepal
Great Himalayan Trail