Technical reports include national or international reports by university departments, institutes, private industry, or government agencies and laboratories, so you will have to know which government, university or institute publishes them. They are hard to find - there is no central place to search for them. Of course, you can always try searching Google or Bing using the term "technical report" AND the publishing body name.
The following six databases are fairly broad in their coverage. Begin here if you are trying to cast your research net widely:
The following databases are more focused or specialized. They will be more helpful if you know the specific subject area you are searching for, such as computer science or cold-weather engineering: