Depending on your business model and how you manage your receivables, your Source of Receivables may be:
A loan management system
An invoicing or accounts receivable platform
An ERP system
A payment processor
A marketplace or commerce platform
A billing system
A subscription management platform
A servicing platform
An internal database or proprietary system
In general, any system that reliably records and maintains information about your receivables and their payment activity may qualify as a Source of Receivables.