Expensive and imo not worth it. You can use marketing platforms like Experian to do so. My advice would be to instead put that budget into TOF marketing and tighten up your email marketing funnel.

If you want to pay I would invest in paid ads promoting a freebie that's of value for them to sign up. You can go as little as £5 a day ad spend. I just put some ads live for a client who wants quicker growth so spending £20 and within 2 days has had 50 sign-ups at 80p a sign-up. These people are now on their list to nurture for other offers and services.

Thanks for all your messages! I really appreciate it. Looks like everyone agrees it is a bad idea. Thanks as well for the suggestions!