supservers(8) is available as "man supservers" or under
~ftp/pub/NetBSD/NetBSD-current/othersrc/sup/supservers.8
You can also take a look at a README I put together once
for setting up a sup-mirror:
~ftp/pub/NetBSD/sup/cfg-files/mirror/README
Cheerio
-- 
The cost of living hasn't affected its popularity. (Mark Twain)
current	release=doc host=netbsd.wifo.uni-mannheim.de \ "NetBSD-current mirror"
hostbase=/mit/ftp/pub/NetBSD base=/usr prefix=/usr backup delete use-rel-suffix