One idea: develop some useful information product (a report, an ebook, a newsletter, etc.) that would be relevant and useful to the folks you're interested in,

One idea: develop some useful information product (a report, an ebook, a newsletter, etc.) that would be relevant and useful to the folks you're interested in, then give that product away for free online in exchange for an email address. Then treat the incoming email addresses as leads -- follow up with them individually, don't just dump them into a mailing list and blast spam out to them.