Typically, you swap them their email address for something they'd presumably want. Instant download of a white paper, one-month free course delivered over email about $FOO, etc.

Patrick already mentioned the best way to get email addresses.

However, for my use case (i.e. emailing cold contacts) you usually don't get their address handed to you. I just looked them up with a combination of Yellow Pages and Google-Fu.

Now that I think of it, I could probably ask the people on my list for interviews. Thanks for the idea, Patrick :-)