Many countries make it compulsory for hostels to submit this information to their Tourism Office each month. So ask them.
1. The Tourism Office may only have info for hotels. So you can request they make themselves unwelcome to hostels by demanding hostels to submit this info. Once the info is accessible to all then your competitive advantage may be lost. But never assume that your competitors will use the info as in reality most hostel owners do not do all things to make their hostel more profitable.
2. The Government statistics office will also have info but that may not look realistic.
3. Ask the agents you give allocation to if they will share this info with you.
4. Then consider your feeders like train and bus services, Hostels and city info as above for neighbouring countries.
5. Then think outside the box and talk with your local Alcohol Suppliers, Tour operators, Concert/Event Organisers.
6. Read info put out by STA, Contiki Tours and other youth orientated agencies.
I once found out the average occupancy rate of hotels in London and Stockholm and compared them to hotels in my city. From that I then worked out an average occupancy rate for a hostel in my city. This sort of info helps an understanding of the desperation for full occupancy in some cities because gaining the info on profit per booking is a lot more difficult to obtain. Then you need to read books that discuss the relationship of square meters per guest and the profit per square meter. Then relate that to duration of booking and the massive jump in profit from long term guests ........