There are plenty of ball end nylon strings out there, including the usual suspects -- Martin and D'Addario. I use the hard tension ones to better approximate the fretting resistance of steel strings. I've even been known to harvest the little ball ends from old steel string sets, and use those on tie-end type nylon strings.
http://www.musiciansfriend.com/acces...guitar-strings
http://search.juststrings.com/search...+nylon+strings
http://daddario.com/DADProductDetail...2-14c7fc3ae93c