Beer.
You think I'm kidding but if you go to a bar, there are lots of people who will test whatever you put in front of them for a free drink. If you want to keep your costs down even further then go during happy hour.
Is that "user testing" no... not in the sense we think of it today. Is it testing of the usability of your product: yes it is. Im guessing that most apps issues that are more likely the latter and not the former. Basic things that you think are clear that really aren't (your too close to the problem and the solution). If you explain, point and guide through the same aspect of your feature 10 times in 10 tests people might not get it! These are the problems you might want to solve, dumb it down, change it up, make it better.
Does this work? Yes, as long as you can moderate. If you can you be neutral and friendly and avoid "traps" that will alter your results. Saying things like "MY app" or reacting personally will alter your results, and learning when to "guide them" through a frustrating point and when to let them puzzle through it is a bit of an art (this too offers clues if your guiding people at the same place). For me personally a drink or two helps ME in the process...
I have also discovered that this process works very well with pairs of people. Two friends already have a rapport and will happily talk to each other to try to get through something problematic. One can encourage or discourage the other and their dialog can be enlightening. Letting them play together with your app and drink free drinks can give you a LOT more insight than the next three individuals testing. Anything more than two people becomes problematic so look to avoid large groups.