Technical reports are part of the gray literature. They:
In addition to the resources here, one of the easiest ways to find technical reports is to identify the organization or university that generated the research and go to their website. If the report is not free to download, use the Libraries' Interlibrary Loan service to get a copy of the report.
Many of the more technical and scientific of the databases that the Libraries subscribe to contain technical reports as well. Search for your topic AND "technical reports" to find. This strategy works online as well (including the use of quotes to search for the phrase technical reports rather than each word separately). Many universities now host Institutional Repositories where faculty archive their research. They can be a good source of technical reports and other research.