I'm trying to figure out a way to get all active subscribers for an active billing plan through the paypal API but unfortunately I can't really find how to do this from the documentation.
I have tried to look into the /v1/billing/subscriptions/{id}/transactions endpoint but for that I'd have to know the subscription-ID first which I'm not sure where to get.
Any help would be greatly appreciated!
You're expected to keep track of active subscriptions in your system. There's no API to list active subscriptions, nor active subscribers.
Since you've apparently not stored this information in your own database at subscription creation time, you may find the 'Reports' tab in PayPal.com useful. You can download an activity log in CSV format, and import it.