Is it okay to ask a supervisor for the email addresses of their current students?
In many places it would likely be viewed as improper. I suggest that you don't do that. Instead, ask the supervisors to pass on a note from yourself to their advisees/students/whoever in which you describe what you want to do and providing contact information for yourself. Make a plea for participation, if you like.
But giving your contact information indirectly to others is vastly different from &quot;harvesting&quot; contact information indirectly.
I would treat your request as stated as improper and wouldn't comply. But I'd be happy to let people know of your project.