Yep, that's a common tricky situation for UXers!
If you're focussed on a commercial product and wanting people to participate for free, are you able to recruit them directly on any of your existing systems, like the product itself, a list of past customers, or the company website? For example, services like ethn.io and SurveyGizmo let you add a popup to existing sites to ask visitors if they'd like to participate in research. Like @enlightened_06 said, if you're not incentivising/compensating participants, it'll need to be super short.
Other services like userzoom.com, userfeel.com and usertesting.com will help you find participants matching your requirements, and are cheaper than using a specialist recruiter (which might be upwards of $100 per participant + incentives). I've known people to use Mechanical Turk and CraigList too (with mixed results).
Heading out in person to where your target users might be is also a solid option (and probably my strongest recommendation, if it is possible for you). It's hard work, but the face-to-face interaction will yield great results. Is that an option for the kind of stuff you're researching? If your target is broad, you might still use family, friends, co-workers and people at the local cafe to get some general opinions and input (but rarely much that is specific).
Some other tools that might be helpful: