You won't find much when searching for "mac" because as @pbacot observed, the vast majority of extensions/plugins run equally well on both platforms. In that case, they don't break out "mac" as a visible keyword anywhere that the search would find. You should search for the functions you need and then verify Mac-compatibility rather than the other way around. Plugins usually state both the OS and the SketchUp versions the author has tested for compatibility. Given the major changes in SketchUp and the version of Ruby it uses since SU 8, many authors no longer support SU 8. That's a bigger factor than Mac vs Windows.