There are three commentaries on the Bhakti-rasāmṛta-sindhu of Śrī Rūpa Gosvāmī. They are by Śrī Jīva Gosvāmī, Śrī Mukunda dāsa and Śrī Viśvanātha Cakravartī Ṭhākura. Are the original sanskrit commentaries available somewhere?
Yes, you can find them here:
- https://archive.org/details/p_BRS (has only Viśvanātha Cakravartī's commentary)
- http://scsmath.org/publications/pdfs-IndianLanguage/SriBhakti-rasamrita-sindhu.pdf (has only Jīva Gosvāmī's Commentary)