There are a few different ways to try and get in touch with large institutional shareholders when they are looking to sell or pledge stock from their portfolio. One way is to reach out to them directly by email or phone. However, this can be difficult, as these types of investors may be difficult to contact or may not be interested in speaking with individual investors.

Another way is to monitor the filings that these institutions make with the Securities and Exchange Commission (SEC). Many institutions are required to disclose when they buy or sell significant amounts of stock, and these filings can include contact information for the institution's representative.

Additionally, you can also try reaching out to the investor relations department of the company whose stock is being sold, since they might be able to provide you with contact information for the institutional investors that hold shares of that company.

Lastly, you can also use stock screening and monitoring tools, like those available from financial data providers, to track large institutional shareholders and their activity in certain stocks, which can help you identify when they are buying or selling shares, and allow you to reach out to them accordingly.

It's important to note that, these institutional investors are professional and have a legal and ethical obligations to follow, they might not easily share their information or entertain unsolicited communication, it might be good to have some context on how you could add value to them as well.