If B2C and you and your friends can be buyers/users:
Directly recruit friends of friends (so they don’t know you)
Post the request on your social networks and your friends’ social networks
Where applicable, put up signage with a landing page URL at the watering holes where your target users hang out (e.g. at a preschool if recruiting parents of young children)
If B2B or you/your friends aren’t buyers/users:
Reach out into your network to see if you know someone who knows someone
Use LinkedIn to find suspects (and how you are connected)
Tap into alumni networks of your affiliated universities
You can also use a research service who can recruit subjects for you (works better for B2C). This costs money.
Stopping people in the mall or putting up flyers in the local library is generally not a great way to recruit subjects.