1. General: Contractor Associations - see here. Contractor association websites provide a wealth of roofing resources, including lists of their members. Most contractor associations have professional ethics requirements for their members, which makes them a pretty good place to start if you're looking for a contractor. Members tend to be larger roofing companies who are more typically focused on commercial roofing, so they may not always be the best place to find a roofer for smaller residential jobs.
4. Contracts: The zlien (Mechanics Lien & Construction Payment Platform) website provides state-by-state information about US mechanics lien and lien waiver rules. (Scroll to the bottom of the home page).
5. Licensing: To verify a contractor's license, and to find out the roofing contractor licensing requirements in your state, see Roofing Contractor Licensing and License Verification.
6. Solar Energy: Find a Certified Solar Roofing Professional (CSRP) at the website of Roof Integrated Solar Energy (RISE), which is an organization "created to provide a means of evaluating and certifying solar roofing professionals to support the widespread use of rooftop solar energy".