For free? Probably not.

Wireguard has been audited by some University groups, maybe contact one of them:

• https://www.cerias.purdue.edu/assets/pdf/bibtex_archive/2021-3.pdf
• https://courses.csail.mit.edu/6.857/2018/project/He-Xu-Xu-WireGuard.pdf

You can try to apply for a grant at Mozilla: https://www.mozilla.org/en-US/moss/secure-open-source/
Their list of "audits we've completed so far" ends in 2019, though, so no idea if they still have money for this.

Otherwise, sometimes governments or hacking contests, like Pwn2Own, do audits/pentests, but you pretty much just have to be a well-known open-source project either way...

Ah, I think I may have used the wrong terminology (though thank you for the respobses). What I'm really looking for is an organisation that provides public reviews of FOSS codebases and assesses their privacy and security. Is that a thing?