What the experts say
Linn Vizard created a great guide that gives an overview of different ways to recruit research participants.
Here are the different ways she listed in her guide with a personal comment:
Recruiting Service: when you ask a company to recruit research participants for you (for example, Testing Time)
Network or Open Recruiting: when you do all the work and share with the world that you're looking for research participants (for example, posting on Linkedin a call for participation, sending an invitation in niche communities, etc.)
Intercept Recruiting: when you go on in a location where the research participants already go and try to "intercept" them to ask them questions (for example being next to the reception of the service you are improving, asking people in the streets, etc.)
I obviously recommend that you read Linn's full guide as it comes with a list of pros and cons and her personal point of view on her experience. Also, the comments in this guide are a great space for learning.
My two cents
Start with what you have
Use your own clients; most companies already have clients and want to improve things for their existing clients. In that case, just ask directly your clients, send them an email or call them.
Recruiting services are worth it
If you have the budget and are looking for people outside of the customers you already have, using a good recruiting service like Testing Time is, for me, a no-brainer.
They really support you and do the hard and boring work for you so that you can just show up on the call with the research participant.
For a large organization, that kind of stuff really pays for itself, as it allows you to focus on tasks that are more important for the company.