Is there a website, platform, to find beta testers for OSX applications?
There is a platform by Stack Exchange. From Abby Miller's post:
We've got a community of talented iOS and OS X developers hanging out on Stack Overflow, and a committed community of power users and enthusiasts hanging out here on Ask Different. Doesn't it seem natural to provide a way to help people interested in testing new software and developers in need of alpha and beta testers find each other?
This program will match developers with potential testers. See the post above for more information and to sign up for the program.
You could use Amazon's Mechanical Turk to set up some test scenarios (as "HITs" - Human Intelligence Tasks) if you're willing to shell out a couple ducats.