First step is that you need to list your competitors. Most of them have happy customers list, or references list on their website. After that, you should look into those lists and make your leads.
This process is a little bit more complicated than the others. Because you need to convince these people to buy your product or service. It is always expensive to make this kind of change for a company. You should show them the benefits and positive differences of your product/service and how it actually differs from the one they already have.
Why should we segment our leads by competitors?