If you are not already familiar with Capterra, it is a paid software directory where people can leave a review for any software listed there. It is one of the oldest, trusted, and leading directories out there.
First, search your competitor on the website https://www.capterra.com.
Let's take Bitly for instance since it is my competing product.
Click on the result in the results page which will take you to the product page. https://www.capterra.com/p/172575/Bitly/ in this case.
Go to reviews and Sort by lowest rating to find disgruntled customers.
Open up the debugger in Chrome or Firefox and reload the page. In Firefox open up the Network tab, Select XHR and click on the link similar to this (https://www.capterra.com/spotlight/rest/product?productId=<Your ID>&countryCode=US)
Go to the response tab on the right pane. Click on reviews.
Expand the reviewer array and copy all the relevant fields. Some fields like Company name are not displayed on the page but, appear in the response here. This is what you will be most interested in.
Head to Linkedin and search with these parameters. Google search also worked for me.
Then, Message/Email them and SELL your product!
What's great is that this publicly available information and does not require a login.
PS: If you want to increase your click-through rates, checkout my product Blanq.