Much of the information that you need for a threat assessment can be found on the free web. Here are some search tips:
“types of threat assessment” finds only sources that have those words in that same order
Terrorism threat assessment site:gov finds only government sites that contain the words Terrorism threat assessment.
Most cities and towns have data available through their municipal web sites. Here are some other resources.
Ports of entry – airports, harbors
Transportation infrastructure – highways, roads, bridges, rail lines, tunnels
Other federal facilities (buildings, research labs, nuclear plants)
Government buildings (city, state, federal)
Stadiums, arenas, convention centers
Other large buildings (high-rises), landmarks, monuments
Communications and technology infrastructure
****Check out this site about dealing with threat assessments to specific buildings: