Self-sourcing while a great temp staffing strategy to reduce cost and increase quality can only be a alternative to staffing agencies and never a full replacement.
Self-sourcing is a form of recruiting and occurs when a preidentified contractor and hiring manager get matched almost without any human interference- through talent pools and open job postings. Obviously not all candidates are actively looking for job and cant always have be on top of all jobs, so the likelihood of these matches happening are infrequent.
For the bulk of requirements, staffing agencies and their recruiters will still be required to reach out and contact candidates, curate them, submit them, schedule interviews, convince them, onboard them and pay them. This is especially true if ther requirements are urgent as they usually are and occur at high volumes.