Research reports are generally written and published by organizations. They might be made freely available by the organization on its website, or they might sell print copies (or access to online versions) to libraries.
Don't confuse these reports with research articles published in journals. Reports are more like a book, often hundreds of pages long.
There's no single best place to search for these, but there are three good options: the Library Catalogue a database, and the web (Google).
In the Library Catalogue, it's not always easy to determine what's a research report, as opposed to a book. Look for these as hints that it's a report:
These are some examples of reports you can find in the Library Catalogue. Many more are available.
To find them, use a keyword search for your topic, then look through the results; they're not identified as reports in the Library Catalogue, so you can't limit to reports. Use the criteria above to recognize reports.
The Canadian Documents Collection includes thousands of research reports from Canadian organizations. The majority of these are also listed in the Library Catalogue, with direct links to the database, but you can search the database directly.
Many organizations provide access to their research reports via their website.