What we've done is simply put the app in the app store, for free, as beta. Generally we get a bunch of installs. After a long a laborious bug cleaning and feature-adding process, we start selling it. It seems to work pretty well, since people don't expect much from you when you're explicit that you're in beta (and it's free), but they can give you analytics about how your app's doing.
And, if you're lucky enough, you'll get someone who needs exactly what your app delivers, and will be willing to work with you to iron out and test all the bugs. You get a free, comprehensive tester, they get an app that steadily improves. It's a win-win all around.