I got asked this on Quora recently:

Question: What are the ways to gather contacts for email marketing? Should I buy a list or grow the list organically? What are the risks in buying an email list? Are there any authentic sources for genuine contacts?

Answer: Buying a list would be disastrous for your business and a waste of money.
First you will be violating the CAN SPAM Act in the US. Or CASL in Canada.

Worse your domain is at risk to be blacklisted as a spammer ruining all future chances of developing good email practices

But let me tell you how to build a great valuable list quickly.

1) Data-mine all existing relationships from your company’s employees personal business lists and get them into a smart automation system like Infusionsoft

2) Invite to your list through Linkedin using existing employee relationships.

3) Develop a brilliant lead magnet  (a valuable asset you can offer in return for an opt-in from a contact). Post on your company website and all social media outlets.

4) Incentivize employees to help grow your list. Run an internal contest.

5) Develop a system in your smart automation system that lets your sales employees send an email to people they have met for lunch offering your lead magnet.

That’s a start but you can get inventive from there.