I would pick a very specific niche (landscaping companies for high end architecture firms) or focus on local businesses in a category (architecture firms) and try to answer that question for myself. I'd connect with a few and ask them how they have done RFPs in the past and if they have any current projects open for bid.

You can search for 'how to find new and available RFPs' and the search results will explain services you can pay to get this information.

Free - Hustle and reach out to businesses

Paid - Directories of open RFPs