Where is the best place to obtain mailing lists?

[Craig Ignatowitz]

Are there any worthwhile companies out there? I work for a website. We are looking for new ways to contact potential members. We are specifically looking for Agents/Brokers/Attorneys etc. All must have verifiable REO experience.
We have bought several lists from a few different companies with little or no ROI.
Does such a source truly exist?
Thanks for any input