Unbounce’s API does not currently support Convertables. We may add this in the future, however it is not on our near-term roadmap.
There is a workaround for getting Convertable leads by using the Pages endpoint (see documentation here). You’ll just need to substitute the {page_id} with your {convertable_id} and you should be good to go.
Disclaimer: This is not a documented access point as we may decide to use a different endpoint to support convertables in the future. Any changes we make to the API may break this functionality so use at your own discretion.