Yes they do not allow solo permits. You may get in touch with an agent however, who can club you with other group for the permit. If you are worried about the costs for booking an agent (as they will offer you an entire package), just ask them to book the hotels for you, which you can choose online and let them know. Permit and immigration will be taken care by them. Alternatively: If you are entering by road (the Phuentsholing border), stay overnight in Phuentsholing (doesn't need a permit). Ask the hotel authorities to help you out or go to the immigration and find your own group, with similar itinerary to club with for immigration purpose.