I tried a variety of things including LinkedIn ads, LinkedIn groups, posting on bulletin boards and e-mailing individuals I found online. I had two very specific user profiles.
For each profile we tried several strategies until we found one that worked. The successful approach was different for each of our user types. For one, I found an online bulletin board with an active membership (had to pay $20 to join) and one post produced about 8 e-mails from people with the profile I needed. You can also ask these recruits for referrals.
For the other, we found a niche professional role in higher education that was interested in one of the products I've designed. We also contacted smaller institutions. This combination of role and institutional profile worked for us.