You can usually find academic technical reports on the website of the department or research institute that produced the report. Here are 2 search engines that may also be helpful for locating academic technical reports.

You can usually find corporate technical reports on the website of the organization that produced the report. NOTE:  You may find that reports from corporate sources cost money. Try requesting these publications via Interlibrary Loan.

Select collections of Technical Reports (many from major government agencies).