Many of us don't make good use of the infrastructure of mailing lists that have developed around us over the past 35 or so years—since the days of the punch cards.
When you work with one, be sure to request the full datacards from them and check out the optional selects that the list includes. They will always be eager to rent a large file to you, but generalizing from many past campaigns, I can say that you rarely benefit from mailing outside the 1 or 2 segments that are most appropriate, within a much larger list or masterfile.
A good datacard typically provides one or two pages of description regarding profile and source of names: a box on the upper left will show your incremental CPM for using specific selects. For a sample, visit MGI or InFocus, two of the two major managers working in the association space.
Of course I'm assuming that you deal with smaller volumes of rental names and much narrower selects than do our peers in philanthropic and commercial marketing. If so we are less profitable customers-it takes more to make us happy and we generate less gross revenue for the list brokers & managers. If not, consider subscribing to SRDS for a year—it's a sensible investment that will give you some very large catalogs of lists that, at least in my case, are resources I have used for years. Counts may change, sometimes list managers will, but enough lists stay on the market in the same place to give you a good starting point when shopping for prospect lists.