There are several synonyms for the phrase "private sources." One possible replacement is "confidential sources," which implies a level of secrecy or discretion. Another option is "non-public sources," which suggests that the information is not widely available or accessible. "Undisclosed sources" can also be used to describe information that is not public knowledge or has not been revealed. Similarly, "unidentified sources" can refer to individuals who have provided information without disclosing their identity. Finally, "off-the-record sources" can indicate that the information is not intended for publication or attribution to a specific person.