If you type in that search term in Google you will get a bunch of agencies claiming they build custom lists.

But the real question is how can you be sure they really do?

Here are some signs to look for:
1. Feel of the website - does it look professional or it's a simple page with no contact information or indication of real people. Read their blog, does it sound like they know their stuff?
2. Case studies - do they have real clients, not just fake names. Search the people who gave testimonials on Linkedin and ask if that's true. Don't be lazy!
3. Check their social media profiles, is it a real agency or just some guy with a scraping bot.
4. Get on a short call with them and see if it's a fit.

The best way to make sure you are getting a high-quality service is to ask for a test sample. At least that is how we at Market Republic onboard new clients. If you want to find out more just google us with prospect list building combined.