I know this issue has been brought up a few times
and I seem to recall someone using some mirrors in russia, but unfortunately I don't recall the details.
This issue has been mentioned in the mailing list back in 2019 and apparently there was this mirror in china
https://mail.gnu.org/archive/html/bug-guix/2022-05/msg00233.html
Since this has been mentioned before I think it is worth asking in IRC and the guix mailing list for an update.
If all that fails you could try bootstrap from source - but I don't know if source downloads are blocked too. And it would take a long time to build (assuming all goes well). This is done by passing --no-substitutes.