You can do this with school districts, entire state governments, and different watchdog groups. If you join a local chamber of commerce, one of the perks mailing list is a member directory. Again, you can market the fact that you are a local business as well and show how you can benefit them. The National Association or Realtors has a list of all registered realtors. You can also get that list by state.
Just think of how powerful that list can be if you are selling a product or service for that niche. Finally, you can ask you local librarian for help. Larger libraries mailing list tend to subscribe to many catalogs that can contain lists of individuals in your niche. There is not free lunch here, of course, and you will have to spend the time to copy and type out all of the names on the list and into your mailing software, but at least you will own your own list after that.
The most important part of direct mail is having a qualified list and then you need to have an excellent sales letter to close the sale on your targeted, tested product. If you can get all three of those parts in line, you will have a very high closing mailing list ratio compared to mailing to a paid list. Publishers know they can save a lot of money joining others in joint or cooperative mailings.