https://www.openbsd.org/ftp.html, second on the list:

https://cloudflare.cdn.openbsd.org/pub/OpenBSD/

But it isn't happy, it's been this way for ~24 hours.

Example:
ftp https://cloudflare.cdn.openbsd.org/pub/OpenBSD/7.5/amd64/SHA256.sig
Trying 104.17.248.92...
TLS handshake failure: handshake failed: error:14004410:SSL routines:CONNECT_CR_SRVR_HELLO:sslv3 alert handshake failure

Used 3rd party tests to check, they confirm
  https://www.isitdownrightnow.com/cloudflare.cdn.openbsd.org.html
  https://downforeveryoneorjustme.com/cloudflare.cdn.openbsd.org?proto=https

tux2bsd