This is something I always search for which doesn’t seem to be very clear from the initial results. The mirrors included below are South Africa specific, so if you are not in South Africa then replace the hostname with something more appropriate for your location.

Add these to /etc/make.conf:

MASTER_SITE_BACKUP?=    \
	ftp://ftp.za.freebsd.org/pub/FreeBSD/ports/distfiles/${DIST_SUBDIR}/

MASTER_SITE_OVERRIDE?=  ${MASTER_SITE_BACKUP}

Mirrors:

See this post for a far more detailed set of MASTER_SITE make variables. This references a lot more of the locally mirrored content.