Richard Zadlicky provides the following list in:
<20091024212559.GA19140@linux-m68k.org>

I checked a few, starting on top of the list at
  http://www.xemacs.org/Releases/index.html#Beta-Download 
down to the Netherland entries - results:

http://ftp.au.xemacs.org/pub/xemacs/beta
http://ftp.at.xemacs.org/editors/xemacs/beta
http://ftp.cz.xemacs.org/MIRRORS/ftp.xemacs.org/pub/xemacs/xemacs-21.5
http://ftp.de.xemacs.org/pub/ftp.xemacs.org/tux/xemacs/beta
http://xemacsftp.hkmirror.org/beta
 - completely dead, server configuration errors, content not existent etc

xemacsftp.digimirror.nl
- DNS error, looking quite dead

http://ftp.br.xemacs.org/pub/xemacs/xemacs-21.5/ - latest 21.5.24
http://mirror.aarnet.edu.au/pub/xemacs/beta/- latest 21.5.28
http://ftp.kr.xemacs.org/pub/tools/emacs/xemacs/beta/ - latest 21.5.28