There are many verses in the Srimad Bhagavatam and the Brahma-Vaivarta Purana which are used by Krishna-centric Vaishnava traditions to show that Lord Krishna is the source of all incarnations, including of Lord Vishnu. Some of such verses are:
ete cāṁśa-kalāḥ puṁsaḥ kṛṣṇas tu bhagavān svayam indrāri-vyākulaṁ lokaṁ mṛḍayanti yuge yuge (Srimad Bhagavatam 1.3.28)
English Translation: All of the above-mentioned incarnations are either plenary portions or portions of the plenary portions of the Lord, but Lord Śrī Kṛṣṇa is the original Personality of Godhead. All of them appear on planets whenever there is a disturbance created by the atheists. The Lord incarnates to protect the theists.
narayanas tvam na hi sarva-dehinam atmasy adhisakhila-loka-saksi narayano ’ngam nara-bhu-jalayanat tac capi satyam na tavaiva maya (Srimad Bhagavatam 10.14.14)
English Translation: Are You not the original Narayana, O supreme controller, since You are the Soul of every embodied being and the eternal witness of all created realms? Indeed, Lord Narayana is Your expansion, and He is called Narayana because He is the generating source of the primeval water of the universe. He is real, not a product of Your illusory Maya.
I am not mentioning the verses from the Brahma-Vaivarta Purana as there are too many to quote.
In addition to the these two Puranas, Gaudiya Vaishnavas use the Brahma Samhita, the famous text containing Lord Brahma's prayers to Lord Krishna that was discovered by Chaitanya Mahaprabhu in Adi-Keshava Temple, to reiterate their point of Lord Krishna to be the Original Supreme Personality of Godhead.
So are there any scriptures which say Lord Rama is the source of all incarnations? I have heard that Ramanandi Vaishnavas believe Lord Rama to be the source of all incarnations. Do they have any scriptural evidence to support their view?