You build a directory on your website, then includes their business details in your directory.
You find their details in various general business directories in your country.
You can start by inluding the first 50-100 companies for free, then the rest see that you have other businesses that "paid" for inclusion, much easier this way. But as always, you need to work on your phone skills, it´s work, but it works.
Most companies that you contact, does not check whois and stuff to see if your website is new, do some free as I wrote above first, then charge a small price for the next 2-300, after that raise your price a bit.