A few weeks ago I contacted Internet Solutions and they are now kindly hosting a local pkgsrc distfiles mirror. To use it, simply add the following to your /etc/mk.conf:
MASTER_SITE_OVERRIDE=ftp://ftp.is.co.za/NetBSD/packages/distfiles/

If a file isn’t found at the local mirror, the pkgsrc fetch Makefile target will fetch it from another (non-local) mirror.