A good starting point can be official documents from governments, such as census data or strategic plans. In addition, documents, studies and reports from institutions working in the town or city, e.g. NGOs or UN Agencies, can be very helpful sources of information. Municipalities, utilities or private local service providers may keep useful records relating to their activities such as design documents, legislation and bylaws, yearly reports, licenses, treatment plant registers, etc.
We also recommend that you read through previous SFD reports produced for a city or town in your country, as this may highlight data sources which you can also use. SFD reports are available via http://SFD.susana.org/SFD-worldwide.